Boiler

ABSTRACT

First, an emission amount of nitrogen oxide can be decreased to zero as much as possible and, an emission amount of carbon monoxide is decreased to a permissible range. Second, energy saving by combustion at a low air ratio close to 1.0 is realized. Third, air ratio control is performed stably in a combustion region at a low air ratio. Provided is a boiler, including: a premixed burner; a water pipe group for, by heat exchanging with a gas produced by the premixed burner, suppressing a temperature of the gas to thereby suppress a concentration of nitrogen oxide to a predetermined value or less; an oxidation catalyst for oxidizing carbon monoxide contained in the gas after the passage through the water pipe group by oxygen and for reducing nitrogen oxide contained therein by carbon monoxide; and an air-ratio adjusting device for adjusting an air ratio of the premixed burner, in which: the premixed burner and the water pipe group have such characteristics that when the air ratio is a set air ratio, a concentration ratio of oxygen, nitrogen oxides, and carbon monoxide in the gas on a primary side of the oxidation catalyst becomes a predetermined concentration ratio; the oxidation catalyst has such characteristics that when the concentration ratio is a predetermined concentration ratio, a concentration of nitrogen oxides on a secondary side of the oxidation catalyst is substantially zero while a concentration of carbon monoxide is decreased to substantially zero or a predetermined value or less; and the predetermined concentration ratio is kept to be constant by being controlled to the set air ratio by the air-ratio adjusting device.

TECHNICAL FIELD

The present invention relates to a boiler applied to a water-tubeboiler, a vapor boiler, a hot water boiler, and the like.

BACKGROUND ART OF THE INVENTION

Generally known principles of suppressing NOx emissions include thesuppression of flame (combustion gas) temperatures and a decrease inretention time of combustion gas at high temperatures. As such, varioustechnologies are available for decreasing the emission of NOx byapplying these principles. Various methods have been proposed and putinto practical use, for example, two-stage combustion, lean-richcombustion, exhaust gas recirculate combustion, water mixing combustion,steam injection combustion, and flame cooling combustion by a water tubegroup.

Moreover, NOx sources relatively small in capacity such as water-tubeboilers are also beginning to be required for a further decrease inemission of NOx due to an increasing awareness of environmentalproblems. In this case, the decrease in NOx generation inevitablyentails an increased amount of emitted CO, thus making it difficult toattain a simultaneous decrease in NOx and CO.

A cause of the above problem is that a simultaneous decrease in emissionof NOx and CO is technically incompatible. More specifically, whentemperatures of combustion gas are abruptly lowered and kept attemperatures of 900° C. or less in an attempt to decrease the emissionof NOx to result in an ample generation of CO, the thus generated CO isemitted before oxidization to increase the amount of emitted CO. Inother words, temperatures of combustion gas are kept higher in anattempt to decrease the amount of emitted CO, thus resulting in aninsufficient suppression of NOx generation.

In order to solve the above problem, the applicant has proposed low NOxand low CO emission technologies for decreasing the amount of CO as muchas possible, which is generated in accordance with a decrease in theamount of NOx generation, and also suppressing temperatures ofcombustion gas so as to attain oxidation of the thus generated CO. Thetechnologies are now commercially feasible (refer to Patent Documents 1and 2). However, an actual value of emitted NOx remains to be about 25ppm in the low NOx emission technologies described in Patent Documents 1and 2.

In order to solve the above problem, the applicant has proposed a lowNOx combustion method in which a NOx decreasing step is conducted tosuppress temperatures of combustion gas so as to give priority tosuppression of NOx generation rather than a decrease in the amount ofemitted CO, thereby keeping the value of the thus generated NOx to apredetermined value or lower, and a CO decreasing step is, thereafter,conducted so as to keep the value of CO emitted from the NOx decreasingstep to a predetermined value or lower (refer to Patent Documents 3 and4). The technologies disclosed in Patent Documents 3 and 4 are able todecrease the amount of emitted NOx to a value lower than 10 ppm, but itis difficult to decrease the amount of emitted NOx to a value below 5ppm. This is due to the fact that combustion characteristics inevitablyentail NOx generation at 5 ppm or greater.

Then, in the low NOx emission technologies disclosed in Patent Documents3 and 4, as shown in FIG. 17, combustion is conducted at a highair-ratio combustion region Z1 where the air ratio is 1.38 or greater.In contrast, at a combustion region Z2 where the air ratio is 1.1 orlower (hereinafter, referred to as “low air ratio”), nitrogen oxides aregenerated in an increased amount, thus making it difficult to attain asimultaneous decrease in the amount of emitted NOx and CO. There is alsoposed a difficulty in controlling a stable combustion due to a possibleoccurrence of backfire where the air ratio is 1 or lower. Therefore, thelow air ratio combustion region Z2 has hardly been subjected to researchand development. In FIG. 17, the lines F and E graphically showcharacteristics of NOx and characteristics of CO on a primary side of acombustion apparatus of the present invention, and the lines U and Jgraphically show characteristics of NOx and characteristics of CO on asecondary side of the combustion apparatus of the present invention.Both of the low NOx emission technologies disclosed in Patent Documents3 and 4 are in principle those in which a burner is used to conductcombustion at the high air ratio region Z1, thereby suppressing thegeneration of NOx and removing the thus generated CO through anoxidation catalyst (Patent Documents 3 and 4).

On the other hand, there is a growing demand for operating boilers at alow air ratio not only to attain a greater decrease in emitted NOx butalso to save energy.

In view of the above-mentioned circumstances, the inventors of thepresent application have been engaged in research and development of acombustion method for decreasing the amount of emitted nitrogen oxidesto zero as much as possible by use of an oxidation catalyst.

Moreover, the method disclosed in Patent Document 5 is known as that oftreating nitrogen oxide-containing gas generated on combustion by aburner.

According to the method of treating exhaust gas disclosed in PatentDocument 5, a burner is used to conduct combustion at an air ratio lowerthan 1.0, whereby oxygen is not contained in combustion exhaust gas butunburned components such as CO and HC (hydrocarbons) are contained, anda nitrogen oxide reducing catalyst is used to reduce nitrogen oxides byunburned components, thereby purifying the nitrogen oxides. Then, air issupplied to exhaust gas after purification, thereby purifying theunburned components by using an oxidation catalyst.

The treatment method disclosed in Patent Document 5 is not a method ofdecreasing carbon monoxide and nitrogen oxides in the presence ofoxygen. Further, according to the method described in Patent Document 5,unburned hydrocarbons are emitted in a great amount, thus making itdifficult to decrease the concentrations of emitted nitrogen oxides andemitted carbon monoxide to substantially zero by using an oxidationcatalyst. Further, the oxidation catalyst characteristics in which anefficiency in reducing nitrogen oxides in the presence of hydrocarbonsdecrease cannot be used. Still further, in a step of reducing nitrogenoxides, a catalyst is used, which is different from that used in a stepof oxidizing unburned components, resulting in a complicated treatment.

Further, a method of purifying nitrogen oxide-containing gas emittedfrom a gas engine is known in Patent Document 6. Patent Document 6describes that nitrogen oxides and carbon monoxide are purified by usinga three-way catalyst, which essentially requires the presence ofhydrocarbons in gas and is applicable only to gas at a theoretical airratio in which no excess oxygen is present. Therefore, the treatmentmethod of Patent Document 6 is not suitable in treating combustion gasresulting from a boiler, which occurs on combustion by a burner andcontains excess oxygen.

Still further, a technology in which an oxidation catalyst is used toreduce nitrogen oxides contained in exhaust gas derived from anincinerator by carbon monoxide is known in Patent Document 7. Accordingto the technology of Patent Document 7, since nitrogen oxides is notreduced in the presence of oxygen in exhaust gas, fuel is burned at anexcessively high concentration (air ratio of less than 1) on primarycombustion, by which exhaust gas is kept deprived of oxygen. Thetechnology in Patent Document 7 is subjected to such restriction thatfuel is burned at an excessively high concentration, thus making itdifficult to find an application for the combustion apparatus such as aburner-equipped boiler in which oxygen is contained in exhaust gas.

[Patent Document 1] Japanese Patent No. 3221582

[Patent Document 2] U.S. Pat. No. 5,353,748[Patent Document 3] Japanese unexamined Patent Application, FirstPublication No. 2004-125378[Patent Document 4] U.S. Pat. No. 6,792,895[Patent Document 5] Japanese unexamined Patent Application, FirstPublication No. 2001-241619[Patent Document 6] Japanese unexamined Patent Application, First PatentDocument 5-38421[Patent Document 7] Japanese unexamined Patent Application, FirstPublication No. 2003-275543

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

Main problems to be solved by the present invention is to decrease theamount of emitted nitrogen oxides and emitted carbon monoxide to zero asmuch as possible or a permissible value by using a simple method andalso to obtain stable effects in decreasing hazardous substances.

Means for Solving the Problems

The inventors of the present application have conducted research forsolving the above problems, finding a point at which the amount ofemitted nitrogen oxides and carbon monoxide is decreased tosubstantially zero in a premixed burner combustion region at a low airratio as close to 1 as possible (the region Z2 in FIG. 17), for whichresearch has been so far hardly conducted for a boiler equipped with anoxidation catalyst to decrease carbon monoxide as described in PatentDocuments 3 and 4. As a result, they have studied causes for which theamount of emitted nitrogen oxides and carbon monoxide can be decreasedto substantially zero, thus obtaining a new finding that a concentrationratio of oxygen, nitrogen oxides, and carbon monoxide on the primaryside of the oxidation catalyst is given as a predetermined referenceconcentration ratio, thereby an oxidation catalyst is used to decreasethe amount of emitted nitrogen oxides and carbon monoxide as close tozero as possible. At the same time, the concentration ratio is adjustedin the vicinity of the predetermined reference concentration ratio,thereby obtaining a new finding that the amount of emitted hazardoussubstances (nitrogen oxides and carbon monoxide) can be decreased tosubstantially zero or a permissible value. The present invention hasbeen completed on the basis of these findings. According to the presentinvention, it is possible not only to decrease the concentration ofemitted hazardous substances to substantially zero but also to attain aremarkable energy savings due to the fact that the above decrease can beobtained at an air ratio as close to 1.0 as possible.

Hereinafter, a simple reference of concentration ratio means aconcentration ratio of oxygen, nitrogen oxides, and carbon monoxide onthe primary side of the oxidation catalyst. The oxidation catalyst mayinclude any known oxidation catalyst or a new oxidation catalyst.

In other words, the inventors of the present application have brokenthrough technical common sense that oxygen is a barrier for reduction ofnitrogen oxides by carbon monoxide on the basis of actions of anoxidation catalyst, as described in Patent Document 7 and used newtechnological approaches for utilizing oxygen to adjust a concentrationrelationship between oxygen, nitrogen oxides, and carbon monoxide on theprimary side of the oxidation catalyst to a predetermined relationship(a predetermined concentration ratio), thus finding absolution for theabove problem.

The above problems include the following auxiliary problems. A firstauxiliary problem is that hydrocarbons, which will inhibit the decreasein hazardous substances (NOx and CO) of the oxidation catalyst, are notcontained in gas generated by a premixed burner. This auxiliary problemcan be solved without use of the hydrocarbon removing device byconducting combustion at which no abrupt cooling is conducted like aninternal combustion engine.

A second auxiliary problem is how to give a concentration ratio of thegas as the predetermined reference concentration ratio. Mere combustionby the premixed burner will not yield the predetermined referenceconcentration. This auxiliary problem can be solved by adjusting theconcentration of oxygen, with the concentration ratio characteristics ofthe premixed burner taken into account, and adjusting it to thepredetermined reference concentration ratio. By using a premixed burner,concentration ratio characteristics at which the concentration ratio canbe adjusted relatively easily while combustion is conducted stably. Theoxygen concentration is adjusted easily and further in a lower air ratioregion by air-ratio adjustment of adjusting a ratio between the amountof a fuel supplied to the premixed burner and a combustion air amount.The air-ratio adjustment of the present invention adjusts not only theratio between the fuel amount and the combustion air amount but also theconcentration ratio, and thus, the air-ratio adjustment of the presentinvention is a novel adjustment different from a conventional air ratiocontrol.

Further, in the adjustment of the concentration ratio, when theconcentration of nitrogen oxides in the gas is too high, the requiredamount of carbon monoxide increases, and the required concentration ofcarbon monoxide may not be obtained stably at the concentration ratiocharacteristics of the premixed burner. The present invention suppressesa combustion gas temperature by a water tube group to suppress theconcentration of nitrogen oxides to be generated, whereby theconcentration ratio can be adjusted easily.

A third auxiliary problem is as follows. Since the present inventionintends to decrease the concentration of emitted hazardous substances tosubstantially zero or a value closer to zero, the concentration ofemitted hazardous substances is increased on change in the concentrationratio due to change in ambient temperature, thus resulting in a failureof obtaining stably decreasing effects. This auxiliary problem can besolved by constantly controlling the concentration ratio. Theconcentration ratio constant-control can be attained by procedures inwhich air-ratio adjusting device are used as means for adjusting theconcentration ratio to detect an air ratio, thus controlling thefeedback of the air ratio.

As described above, the present invention is an epoch-making inventionthat has a remarkable effect of decreasing hazardous substances whichcan be called a zero NOx boiler, using a conventional burner, anoxidation catalyst, and an air ratio control, or even using a technologywhich is a direct extension thereof, and that can realize the energysaving and is friendly to the terrestrial environment.

The invention as described in Claim 1 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and an air-ratio adjusting device for adjusting an air ratioof the premixed burner, in which the premixed burner and the water tubegroup have characteristics in which a concentration ratio of oxygen,nitrogen oxides, and carbon monoxide in gas on a primary side of theoxidation catalyst becomes a predetermined concentration ratio, assumingthat the air ratio is a set air ratio, the oxidation catalyst hascharacteristics in which a concentration of nitrogen oxides on asecondary side of the oxidation catalyst is decreased to substantiallyzero and a concentration of carbon monoxide on the secondary side of theoxidation catalyst is decreased to substantially zero or a predeterminedvalue or less, assuming that the concentration ratio is thepredetermined concentration ratio, and the predetermined concentrationratio is kept to be constant by being controlled to the set air ratio bythe air-ratio adjusting device. In the invention and the followingdescription, “after the passage through a water tube group” includes“after the passage through the entire water tube group” and “after thepassage through a part of the water tube group”.

In this instance, the concentration of nitrogen oxides decreased tosubstantially zero is preferably 5 ppm, more preferably 3 ppm, and stillmore preferably zero. The concentration of carbon monoxide decreased tosubstantially zero is preferably 30 ppm and more preferably 10 ppm.Further, in the following description, the concentration of oxygendecreased to substantially zero is 100 ppm or lower and preferably belowa measurement limit value. Still further, the concentration of nitrogenoxides and that of carbon monoxide lower than a predetermined value meana value below the standard for concentrations of emissions stipulated invarious territories and countries. However, as a matter of course, it ispreferable to set the value to substantially zero. As described above,in the meaning of the standard for concentrations of emissions, a valueequal to or below “predetermined value” may be referred to as“permissible value” or “emission standard value.”

According to the invention as described in Claim 1, the air ratio of theburner is defined as a set air ratio and the concentration ratio of thegas is defined as the predetermined concentration ratio, utilizing thecharacteristics of the premixed burner and the water tube group, wherebythe concentration of emitted nitrogen oxides is decreased tosubstantially zero and the concentration of emitted carbon monoxide canbe decreased to substantially zero or a predetermined value or less byusing the oxidation catalyst. Further, since the concentration of oxygenon a secondary side of the oxidation catalyst becomes substantiallyzero, so the premixed burner is used to conduct combustion at a low airratio, which can realize energy saving. Further, the premixed burner isused to conduct combustion so that hydrocarbon is not emitted in thegas, so the combustion conducted by the premixed burner can becontrolled easily, compared with a method for allowing combustion to beconducted so that hydrocarbon is emitted as in Patent Document 7. Sincehydrocarbon is not contained in gas flowing to the oxidation catalyst,the complicated procedure as in Patent Document 5 is not used, andnitrogen oxides and carbon monoxide can be decreased effectively by theoxidation catalyst, and the predetermined concentration ratio can beadjusted easily without considering the reaction by hydrocarbon.Further, since the predetermined concentration is kept to be asubstantially constant value by the air-ratio adjusting device, thefluctuation in the predetermined concentration ratio by the fluctuationin an outside air temperature can be suppressed, the stable effect ofdecreasing hazardous substances can be exhibited, and means forcontrolling the predetermined concentration ratio to be constant is notrequired separately from the air-ratio adjusting device, which cansimplify the configuration of an apparatus. Further, since a premixedburner is used, the predetermined concentration ratio can be obtainedrelatively easily in a low air ratio region.

The invention as described in Claim 2 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and an air-ratio adjusting device for adjusting an air ratioof the premixed burner, in which the premixed burner and the water tubegroup have characteristics in which a concentration ratio K of oxygen,nitrogen oxides, and carbon monoxide in gas on a primary side of theoxidation catalyst becomes a predetermined reference concentration ratioK0, assuming that the air ratio is a reference set air ratio, theoxidation catalyst has characteristics in which a concentration ofnitrogen oxides and a concentration of carbon monoxide on a secondaryside of the oxidation catalyst are decreased to substantially zero,assuming that the concentration ratio K is the predetermined referenceconcentration ratio K0, and the predetermined concentration ratio iskept to be constant by being controlled to the reference set air ratioby the air-ratio adjusting device.

According to the invention as described in Claim 2, the air ratio of theburner is defined as a set reference air ratio and the concentrationratio K of the gas is defined as the predetermined referenceconcentration ratio K0, utilizing the concentration ratiocharacteristics of the premixed burner and the water tube group, wherebythe concentrations of emitted nitrogen oxides and emitted carbonmonoxide can be decreased to substantially zero by using the oxidationcatalyst. Further, also in the invention as described in Claim 2, othereffects obtained in Claim 1 as described above can be exhibitedsimilarly.

The invention as described in Claim 3 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and an air-ratio adjusting device for adjusting an air ratioof the premixed burner, in which the premixed burner and the water tubegroup have characteristics in which a concentration ratio K of oxygen,nitrogen oxides, and carbon monoxide in gas on a primary side of theoxidation catalyst becomes a first predetermined concentration ratio K1,assuming that the air ratio is a first set air ratio, the oxidationcatalyst has characteristics in which a concentration of nitrogen oxideson a secondary side of the oxidation catalyst is decreased tosubstantially zero and a concentration of carbon monoxide on thesecondary side of the oxidation catalyst is decreased to a predeterminedvalue or less, assuming that the concentration ratio K is the firstpredetermined concentration ratio K1, and the predeterminedconcentration ratio is kept to be constant by being controlled to thefirst set air ratio by the air-ratio adjusting device.

According to the invention as described in Claim 3, the air ratio of theburner is defined as a first set air ratio and the concentration ratio Kof the gas is defined as the first predetermined concentration ratio K1,utilizing the concentration ratio characteristics of the premixed burnerand the water tube group, whereby the concentration of emitted nitrogenoxides can be decreased to substantially zero and the concentration ofemitted carbon monoxide can be decreased to a predetermined value orless by using the oxidation catalyst. Further, also in the invention asdescribed in Claim 3, the effects obtained in Claim 1 and other effectsas described above can be exhibited similarly.

The invention as described in Claim 4 provides the boiler according toClaim 2 or 3, in which a formula for determining the predeterminedreference concentration ratio K0 is given as the following formula (1),the predetermined reference concentration ratio K0 satisfies thefollowing formula (2), and the first predetermined concentration ratioK1 is made smaller than the reference concentration ratio K0.

According to the invention as described in Claim 4, the effects similarto those in Claims 2 and 3 can be exhibited.

The invention as described in Claim 5 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; a sensor for detecting an air ratio of the premixed burner;and an air-ratio adjusting device for controlling the premixed burner toa set air ratio based on a detected signal of the sensor, in which thepremixed burner and the water tube group are configured so that, whenthe air-ratio adjusting device is used to adjust the air ratio to theset air ratio, a predetermined concentration of oxygen, nitrogen oxides,and carbon monoxide on a primary side of the oxidation catalyst, atwhich a concentration of nitrogen oxides on a secondary side of theoxidation catalyst is decreased to substantially zero and aconcentration of carbon monoxide on the secondary side of the oxidationcatalyst is decreased to substantially zero or a predetermined value orless, is obtained. In this Claim and the following Claims, the detectedair ratio (air ratio detected by the sensor) and the set air ratio canbe replaced by a detected air/fuel ratio and a set air/fuel ratio,respectively, or a detected oxygen concentration and a set oxygenconcentration, respectively.

The invention as described in Claim 6 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; a sensor for detecting an air ratio of the premixed burner;and an air-ratio adjusting device for controlling the premixed burner toa set air ratio based on a detected signal of the sensor, in which thepremixed burner and the water tube group have characteristics of airratio-NOx/CO in which, when the air ratio is adjusted to the set airratio by the air-ratio adjusting device, a concentration of nitrogenoxides and a concentration of oxygen on a secondary side of theoxidation catalyst is decreased to substantially zero, and aconcentration of carbon monoxide on the secondary side of the oxidationcatalyst is decreased to substantially zero or a predetermined value orless.

The invention as described in Claim 7 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; a sensor for detecting an air ratio of the premixed burner;and an air-ratio adjusting device for controlling the premixed burner toa set air ratio based on a detected signal of the sensor, in which thepremixed burner and the water tube group are configured so that, whenthe air-ratio adjusting device is used to adjust the air ratio to theset air ratio, the concentration of carbon monoxide on a primary side ofthe oxidation catalyst is substantially equal to or greater than a valueobtained by adding a concentration of carbon monoxide decreased insidethe oxidation catalyst due to the oxidation to a concentration of carbonmonoxide decreased inside the catalyst due to the reduction.

The invention as described in Claim 8 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; a sensor for detecting an air ratio of the premixed burner;and an air-ratio adjusting device for controlling the premixed burner toa set air ratio based on a detected signal of the sensor, in which thepremixed burner and the water tube group are configured so that, whenthe air-ratio adjusting device is used to adjust the air ratio to theset air ratio, a concentration ratio of the gas before flowing into theoxidation catalyst satisfies the following formula (3):

([NOx]+2[O₂])/[CO]≦2.0  (3)

where [CO], [NOx], and [O₂] represent concentrations of carbon monoxide,nitrogen oxides, and oxygen, respectively, and satisfying a condition of[O₂]>0.

According to the inventions as described in Claims 5 to 8, the effectsimilar to that in Claim 1 can be exhibited.

The invention as described in Claim 9 includes, according to Claims 5 to8, the set air ratio of substantially 1.0.

According to the invention as described in Claim 9, the effect ofrealizing energy saving by low-air ratio combustion close to 1 as muchas possible can be exhibited, in addition to the effects by theinventions as described in Claims 5 to 8.

The invention as described in Claim 10 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and an air-ratio adjusting device for adjusting an air ratioof the premixed burner, in which the premixed burner and the water tubegroup have characteristics of air ratio-NOx/CO on a primary side of theoxidation catalyst regarding the gas containing oxygen, nitrogen oxides,and carbon monoxide on the primary side of the oxidation catalystobtained by adjusting in a vicinity of an air ratio of 1.0 by theair-ratio adjusting device, the oxidation catalyst has characteristicsof air ratio-NOx/CO on a secondary side of the oxidation catalystobtained by bringing a gas having the characteristics of airratio-NOx/CO on the primary side into contact with the oxidationcatalyst, and the air-ratio adjusting device controls an air ratio ofthe premixed burner at a set air ratio at which a concentration ofnitrogen oxides on the secondary side of the oxidation catalyst isdecreased to substantially zero in a NOx/CO decreasing region of thecharacteristics of air ratio-NOx/CO on the secondary side.

According to the invention as described in Claim 10, the set air ratiois controlled, whereby the concentration of emitted nitrogen oxides canbe decreased to substantially zero and the concentration of emittedcarbon monoxide can be decreased to substantially zero or apredetermined value or less, using the oxidation catalyst. Other effectsare similar to those in Claims 5 to 8.

The invention as described in Claim 11 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst that is brought into contact withthe gas containing oxygen, nitrogen oxides, and carbon monoxide afterthe passage through the water tube group; and an air-ratio adjustingdevice for adjusting a ratio between a combustible air amount of thepremixed burner and a fuel amount, in which the oxidation catalyst hascharacteristics of decreasing a concentration of carbon monoxide by:when a concentration ratio of oxygen, nitrogen oxides, and carbonmonoxide in a gas on a primary side of the oxidation catalyst at which aconcentration of nitrogen oxides and a concentration of carbon monoxideon a secondary side of the oxidation catalyst are decreased tosubstantially zero is a reference concentration ratio, and theconcentration ratio is the reference concentration ratio, concentrationsof oxygen, nitrogen oxides, and carbon monoxide on the secondary side ofthe oxidation catalyst being decreased to substantially zero; and when aconcentration of oxygen on the primary side is lower than aconcentration of the reference oxygen, concentrations of nitrogen oxidesand oxygen on the secondary side of the oxidation catalyst beingdecreased to substantially zero, and the air-ratio adjusting deviceadjusts a concentration of oxygen on the primary side of the oxidationcatalyst with respect to the concentration of the reference oxygen byadjusting the air ratio based on concentrations of oxygen and/or carbonmonoxide on the secondary side of the oxidation catalyst, and decreasesa concentration of nitrogen oxides on the secondary side of theoxidation catalyst and the concentration of carbon monoxide on thesecondary side of the oxidation catalyst to substantially zero, therebydecreasing the concentration of carbon monoxide or decreasing theconcentration of carbon monoxide to substantially zero.

According to the invention as described in Claim 11, the set air ratiois controlled, whereby the concentration of emitted nitrogen oxides canbe decreased to substantially zero and the concentration of emittedcarbon monoxide can be decreased to substantially zero or apredetermined value or less by the oxidation catalyst. Other effects aresimilar to those in Claims 5 to 8.

The invention as described in Claim 12 includes, according to any one ofClaims 1 to 3, 5 to 8, 10, and 11, a nitrogen oxide generationsuppressing device for suppressing a temperature of the gas to suppressa concentration of nitrogen oxides to a predetermined value or less.

According to the invention as described in Claim 12, since thegeneration of NOx is suppressed to a predetermined value or less, theeffect of being capable of decreasing the use amount of the oxidationcatalyst is exhibited in addition to the effects by the inventions asdescribed in Claims 1 to 3, 5 to 8, 10, and 11.

The invention as described in Claim 13 includes, according to any one ofClaims 1 to 3, 5 to 8, 10, and 11, a feed-water preheater on thesecondary side of the oxidation catalyst.

According to the invention as described in Claim 13, since heatgenerated by the oxidation catalyst can be collected in the feed-waterpreheater, the effect of being capable of realizing further energysaving is exhibited in addition to the effects by the inventions asdescribed in Claims 1 to 3, 5 to 8, 10, and 11. Further, by decreasingthe emission amount of nitrogen oxides to substantially zero, thecorrosion of the feed-water preheater can be suppressed.

The invention as described in Claim 14 includes, according to any one ofClaims 1 to 3, 5 to 8, 10, and 11, a second sensor for detectingabnormality of the catalyst or the sensor; a notifying device; and acontrol device for determining the abnormality based on a detected valueof the second sensor to notify the abnormality by the notifying device.

According to the invention as described in Claim 14, an administrator ofthe boiler is informed of the abnormality of the oxidation catalyst orthe sensor to address the abnormality, whereby the effect of beingcapable of preventing the emission of a great amount of carbon monoxideis exhibited, in addition to the effects by the inventions as describedin Claims 1 to 3, 5 to 8, 10, and 11.

EFFECTS OF THE INVENTION

According to the present invention, the concentration ratio is adjustedto the predetermined concentration ratio by the air-ratio adjustingdevice, whereby the emission amounts of nitrogen oxides and carbonmonoxide can be decreased to close to zero as much as possible by usingthe oxidation catalyst. Further, the decrease in hazardous substancescan be realized stably. Further, the configuration of an apparatus canbe simplified without requiring separately means for controlling aconcentration ratio to be constant. Further, compared with a premixedburner, combustible air and a fuel can be mixed uniformly to be burned,and gas at the predetermined concentration ratio can be generated stablyand relatively easily with less generation amounts of hydrocarbon and COeven in the vicinity of a theoretical air ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A longitudinal sectional view for explaining a steam boiler ofEmbodiment 1.

FIG. 2 A sectional view taken along line II to II in FIG. 1.

FIG. 3 A view showing a constitution of major parts when an oxidationcatalyst given in FIG. 2 is viewed from a direction in which exhaust gasflows.

FIG. 4 A drawing showing characteristics of air ratio-NOx/CO inEmbodiment 1.

FIG. 5 A partial sectional view for explaining a damper positionadjusting device of Embodiment 1, which is in operation.

FIG. 6 A sectional view for explaining major parts of the damperposition adjusting device.

FIG. 7 A pattern diagram for explaining characteristics of a burner andan endothermic device and those of a catalyst given in Embodiment 1.

FIG. 8 A drawing for explaining output characteristics of the sensorgiven in Embodiment 1.

FIG. 9 A drawing for explaining motor controlling characteristics inEmbodiment 1.

FIG. 10 A drawing for explaining the NOx and CO decreasingcharacteristics in Embodiment 1.

FIG. 11 A longitudinal sectional view for explaining a steam boiler ofEmbodiment 2.

FIG. 12 A drawing for explaining motor controlling characteristics inEmbodiment 2.

FIG. 13 A longitudinal sectional view for explaining a steam boiler ofEmbodiment 3.

FIG. 14 A longitudinal sectional view for explaining a steam boiler ofEmbodiment 4.

FIG. 15 A flowchart for explaining a control procedure of Embodiment 4.

FIG. 16 A flowchart for explaining a control procedure of Embodiment 4.

FIG. 17 A drawing for explaining primary characteristics and secondarycharacteristics of NOx and CO in the present invention.

DESCRIPTION OF THE REFERENCE SYMBOLS

-   -   1: burner    -   2: water tube group (heat transfer tube group)    -   4: oxidation catalyst    -   7: sensor    -   8: controller    -   28: air-ratio adjusting device    -   29: damper    -   30: damper position adjusting device    -   34: motor

BEST MODE FOR CARRYING OUT THE INVENTION

Next, an explanation will be given for embodiment modes of the presentinvention. An explanation will be made for terms used in the presentapplication before the embodiment modes of the present invention will beexplained. “Gas” refers to gas before completely passing from a burnerthrough an oxidation catalyst (which can be also referred to as anoxidation/reduction catalyst and hereinafter simply referred to as“catalyst”), and gas which has passed through the catalyst refers to“exhaust gas.” Therefore, the gas includes that in which burningreactions are in progress (combustion process) and that in which theburning reactions have been completed, and can be also referred to ascombustion gas. In this instance, where the catalyst is installed inmultiple stages along the gas flow, the “gas” is defined as gas beforecompletely passing through the catalyst at a final stage, and “exhaustgas” is defined as gas after passing through the catalyst at the finalstage.

A “primary side of the catalyst” is a side where a burner is installedwith respect to a catalyst, referring to immediately before the passageof gas through the catalyst unless otherwise specified, whereas a“secondary side of the catalyst” is a side opposite to the primary sideof the catalyst.

Further, “free of hydrocarbons” does not mean that hydrocarbons will notbe generated at all in a process of burning reactions, but means thathydrocarbons are generated to some extent during the process of burningreactions but hydrocarbons, which reduce nitrogen oxides are notsubstantially contained (a measurement limit or lower) in gas flowinginto the catalyst at a stage where the burning reactions are completed.

Further, an air ratio m is defined as m=21/(21−[O₂]). Note that [O₂]represents the concentration of oxygen in exhaust gas on the secondaryside of the catalyst, but [O₂] used in determining an air ratiorepresents the concentration of excess oxygen in an oxygen excess regionand also represents as a negative value the concentration ofinsufficient oxygen necessary for burning unburned gas such as carbonmonoxide at the air ratio of m=1 in a fuel excess region.

Next, an explanation will be made for embodiment modes of the presentinvention. The present invention is applicable to a water-tube boilersuch as a small through-flow boiler, a hot-water supply system, and acombustion apparatus (also referred to as a thermal component or acombustion device) used in a regenerator for an absorption refrigerator.

EMBODIMENT 1

Embodiment 1 of the present invention is a boiler including: a premixedburner for burning a hydrocarbon-containing fuel to generate gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from gas generated by the premixedburner; an oxidation catalyst for oxidizing the carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and air-ratio adjusting device for adjusting an air ratio ofthe premixed burner, in which the premixed burner and the water tubegroup have characteristics in which a concentration ratio among oxygen,nitrogen oxides, and carbon monoxide in gas on a primary side of theoxidation catalyst becomes a predetermined concentration ratio, assumingthat the air ratio is a set air ratio, the oxidation catalyst hascharacteristics in which the concentration of nitrogen oxides on asecondary side of the oxidation catalyst is decreased to substantiallyzero and the concentration of carbon monoxide is decreased tosubstantially zero or a predetermined value or less, assuming that theconcentration ratio is the predetermined concentration-ratio, and thepredetermined concentration ratio is kept to be constant by beingcontrolled to the set air ratio by the air-ratio adjusting device.Embodiment 1 includes Embodiment Modes 2 and 3 as described below.

EMBODIMENT MODE 2

Embodiment 2 is a boiler including: a premixed burner for burning ahydrocarbon-containing fuel to generate gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from gas generated by the premixed burner; anoxidation catalyst for oxidizing the carbon monoxide contained in thegas after the passage through the water tube group by oxygen andreducing the nitrogen oxides contained in the gas by carbon monoxide;and air-ratio adjusting device for adjusting an air ratio of thepremixed burner, in which the premixed burner and the water tube grouphave characteristics in which a concentration ratio K among oxygen,nitrogen oxides, and carbon monoxide in gas on a primary side of theoxidation catalyst becomes a predetermined reference concentration ratioK0, assuming that the air ratio is a set reference air ratio, theoxidation catalyst has characteristics in which the concentrations ofnitrogen oxides and carbon monoxide on a secondary side of the oxidationcatalyst is decreased to substantially zero, assuming that theconcentration ratio K is the predetermined reference concentration ratioK0, and the predetermined concentration ratio is kept to be constant bybeing controlled to the set reference air ratio by the air-ratioadjusting device.

EMBODIMENT MODE 3

Further, Embodiment Mode 3 is a boiler including: a premixed burner forburning a hydrocarbon-containing fuel to generate gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from gas generated by the premixedburner; an oxidation catalyst for oxidizing the carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and air-ratio adjusting device for adjusting an air ratio ofthe premixed burner, in which the premixed burner and the water tubegroup have characteristics in which a concentration ratio K amongoxygen, nitrogen oxides, and carbon monoxide in gas on a primary side ofthe oxidation catalyst becomes a first predetermined concentration ratioK1, assuming that the air ratio is a first set air ratio, assuming thatthe air ratio is a first set air ratio, the oxidation catalyst hascharacteristics in which the concentration of nitrogen oxides on asecondary side of the oxidation catalyst is decreased to substantiallyzero and the concentration of carbon monoxide is decreased to apredetermined value or less, assuming that the concentration ratio K isthe first predetermined concentration ratio K1, and the predeterminedconcentration ratio is kept to be constant by being controlled to thefirst set air ratio by the air-ratio adjusting device.

The predetermined reference concentration ratio K0 and the firstpredetermined concentration ratio K1 are respectively subjected toAdjustments 0 and 1 as described below by being controlled to be thereference set air ratio and the first set air ratio.

Adjustment 0: the concentration ratio K is adjusted to a predeterminedreference concentration ratio K0 in which a concentration of nitrogenoxides and a concentration of carbon monoxide on the secondary side ofthe oxidation catalyst are decreased to substantially zero.

Adjustment 1: the concentration ratio K is adjusted to a firstpredetermined concentration ratio K1 in which the concentration ofnitrogen oxides on the secondary side of the oxidation catalyst isdecreased to substantially zero and the concentration of carbon monoxideon the secondary side of the oxidation catalyst is decreased to apredetermined value or lower.

Then, the catalyst is characterized in that it decreases each of theconcentration of nitrogen oxides and that of carbon monoxide on thesecondary side of the catalyst to substantially zero when Adjustment 0is made, decreasing the concentration of nitrogen oxides and that ofcarbon monoxide on the secondary side of the catalyst to substantiallyzero and a predetermined value or lower, respectively, when Adjustment 1is made.

In Embodiment Modes 2 and 3, the concentration ratio means a mutualrelationship between the concentration of carbon monoxide, that ofnitrogen oxides, and that of oxygen. A predetermined referenceconcentration ratio K0 of Adjustment 0 is determined by the followingdetermination formula (1), and preferably set in such a manner that itsatisfies the following formula (2), and the first predeterminedconcentration ratio K1 is made smaller than the predetermined referenceconcentration ratio K0.

([NOx]+2[O₂])/[CO]=K  (1)

1.0≦K=K0<2.0  (2)

(in the formula (1), [CO], [NOx], and [O₂] represent concentrations ofcarbon monoxide, nitrogen oxides, and oxygen, respectively, andsatisfying the condition of [O₂]>0).

In other words, the predetermined reference concentration ratio K0 is aconcentration ratio of oxygen, nitrogen oxides, and carbon monoxide onthe primary side of the oxidation catalyst in which the concentration ofoxygen, that of nitrogen oxides, and that of carbon monoxide on thesecondary side of the oxidation catalyst are each decreased tosubstantially zero. Formula (1) is a determination formula to determinethe predetermined reference concentration ratio K0, and formula (2)indicates conditions for decreasing the concentration of oxygen, that ofnitrogen oxides, and that of carbon monoxide on the secondary side ofthe oxidation catalyst to substantially zero. Theoretically, each ofthese concentrations can be decreased to zero under the condition ofK0=1.0. However, experimental results have confirmed that each of theconcentrations can be decreased to substantially zero within a scope offormula (2) and an upper limit of the K0, 2.0, may be a value greaterthan 2.0, depending on characteristics of the catalyst.

When a concentration ratio K on the primary side of the oxidationcatalyst is adjusted so that it is lower than the predeterminedreference concentration ratio K0, in other words, K in formula (1) isgiven as the first predetermined concentration ratio K1 which is smallerthan K0 (Adjustment 1) the concentration of oxygen and that of nitrogenoxides on the secondary side of the oxidation catalyst are decreased tosubstantially zero and the concentration of carbon monoxide is decreasedto a predetermined value or lower. The predetermined value of theconcentration of carbon monoxide is preferably set to be an emissionstandard value or lower (since this value is different depending oncountries, it may be changed in each of the countries). Upondetermination of the predetermined value, it is possible to determineexperimentally the first predetermined concentration ratio K1.Specifically, such adjustment of the concentration ratio K that a valueof the concentration ratio K is given as the first predeterminedconcentration ratio K1, which is smaller than K0, can be made by makingsmaller a ratio of the concentration of oxygen to that of carbonmonoxide on the primary side of the oxidation catalyst than a ratio ofthe concentration of oxygen to that of carbon monoxide, which satisfiesthe predetermined reference concentration ratio K0.

In Embodiment Modes 1 to 3 as described above, first, the premixedburner is used to effect combustion so that oxygen is present on aprimary side of the oxidation catalyst. As a result of the combustion,gas containing oxygen, nitrogen oxides, and carbon monoxide but free ofhydrocarbon is generated. The concentration ratio K among oxygen,nitrogen oxides, and carbon monoxide in the gas on a primary side of thecatalyst is adjusted to the predetermined reference concentration ratioK0 and the first predetermined concentration ratio K1 respectively bythe premixed burner and the water tube group. Then, the gas comes intocontact with the catalyst, whereby carbon monoxide is oxidized by oxygenin the gas and nitrogen oxides are reduced by carbon monoxide. The roleof oxygen in the case where Adjustment 0 or 1 is conducted is to adjustthe concentration of carbon monoxide, i.e., to consume and decrease theamount of carbon monoxide present in an amount more than necessary forreducing the nitrogen oxides to decrease the concentration thereof tosubstantially zero. Due to the contact with the catalyst afterAdjustments 0 and 1, the emission amount of nitrogen oxides in the gasis decreased to substantially zero, and the emission amount of carbonmonoxide is decreased to substantially zero or a predetermined value orless. Further, the concentration ratio is controlled to be constant,whereby the fluctuation in values of the respective predeterminedconcentration ratios K0 and K is suppressed, and the effect ofdecreasing the emission amount of nitrogen oxides and the emissionamount of carbon monoxide can be exhibited. In particular, the controlof setting the concentration ratio to be constant is important fordecreasing the emission amount of nitrogen oxides to substantially zeroin Adjustment 0.

A predetermined reference concentration ratio K0 of Adjustment 0 and afirst predetermined concentration ratio K1 of Adjustment 1 can becollectively expressed by the following formula (3). In other words,when formula (3) is satisfied, the concentration of nitrogen oxides onthe secondary side of the catalyst is decreased to substantially zero,and the concentration of carbon monoxides on the secondary side of thecatalyst is decreased to substantially zero, otherwise the concentrationof carbon monoxide is decreased. In order to decrease the concentrationof carbon monoxide to a value equal to or lower than the predeterminedvalue, the concentration ratio K on the primary side of the oxidationcatalyst is adjusted so that the left-hand value in the formula (3) willbe a value smaller than K0, thereby obtaining the first predeterminedconcentration ratio K1.

([NOx]+2[O₂])/[CO]=K≦2.0  (3)

where [CO], [NOx], and [O₂] represent the concentrations of CO, NOx, andO₂, respectively, and satisfying the condition of [O₂]>0.

EMBODIMENT MODE 4

The present invention includes Embodiment 4 as described below.Embodiment Mode 4 is a boiler including: a premixed burner for burning ahydrocarbon-containing fuel to generate gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from gas generated by the premixed burner; anoxidation catalyst for oxidizing the carbon monoxide contained in thegas after the passage through the water tube group by oxygen andreducing the nitrogen oxides contained in the gas by carbon monoxide; asensor for detecting an air ratio of the premixed burner; and air-ratioadjusting device for controlling the premixed burner to a set air ratiobased on the detection signal of the sensor, in which the premixedburner and the water tube group are configured so as to obtain apredetermined concentration ratio of oxygen, nitride oxides, and carbonmonoxide on a primary side of the oxidation catalyst at which theconcentration of nitrogen oxides in a secondary side of the oxidationcatalyst is decreased to substantially zero and the concentration ofcarbon monoxide is decreased to substantially zero or a predeterminedvalue or less, when the air ratio is adjusted to the set air ratio bythe air-ratio adjusting device.

The set air ratio is preferably controlled to 1.0. The air ratio can bealso controlled so that the concentration of oxygen becomes apredetermined concentration on the primary side of the catalyst, whichis capable of satisfying the set air ratio of 1.0 as a result ofreactions on the catalyst.

In Embodiment Mode 4 of the present invention, combustion is effected inthe burner, with the air ratio controlled by the air-ratio adjustingdevice so as to give the set air ratio. Gas generated on combustion issubjected to endothermic actions by the water tube group. Thereafter,carbon monoxide is oxidized by the catalyst and nitrogen oxides arereduced. As a result, the amount of emitted nitrogen oxides in the gasis decreased to a value close to zero, or 5 ppm or lower. The amount ofemitted carbon monoxide is also decreased.

According to Embodiment Mode 4 of the present invention, the air ratiois controlled by the air-ratio adjusting device so as to give the setair ratio, thus making it possible to obtain a concentration ratio ofoxygen, nitrogen oxides, and carbon monoxide on the primary side of thecatalyst in which the concentration of nitrogen oxides on the secondaryside of the catalyst is decreased to substantially zero.

In controlling a low air ratio, it is difficult to obtain a stablecontrol of the air ratio. However, the air-ratio adjusting device isprovided with electrical control device and/or mechanical control devicefor stably controlling the air ratio, thus making it possible to obtainstable control of the air ratio.

The concentration ratio adjustment on the primary side of the catalystis preferably controlled in such a manner that the concentration ofcarbon monoxide in the gas on the primary side of the catalyst isapproximately equal to or higher than a value obtained by adding theconcentration of carbon monoxide decreased inside the catalyst byoxidation of carbon monoxide (first reaction) to the concentration ofcarbon monoxide decreased inside the catalyst by reduction of nitrogenoxides by carbon monoxide (second reaction).

Hereinafter, the function of decreasing hazardous substances (nitrogenoxides and carbon monoxide) will be described further. The decreasefunction is considered to be performed as follows. In gas free of HC(hydrocarbon), a first reaction for oxidizing carbon monoxide and asecond reaction for reducing nitrogen oxides by carbon monoxide areeffected as main reactions in the catalyst. Then, in a reaction in thecatalyst (catalyst reaction), the first reaction is predominant comparedwith the second reaction in the presence of oxygen. Therefore, carbonmonoxide is consumed by oxygen to have its concentration adjusted basedon the first reaction, and thereafter, nitrogen oxides are reduced bythe second reaction. This description is simplified. Actually, althoughthe first reaction is a competitive reaction with respect to the secondreaction, the reaction between carbon monoxide and oxygen is effectedapparently faster than the second reaction in the presence of oxygen, soit is considered that the first reaction is effected in a first stageand the second reaction is effected in a second stage.

In short, in the catalyst, in the presence of oxygen, oxygen is consumedby the first reaction: CO+½O₂→CO₂, and nitrogen oxides are reduced usingthe remaining CO by the second reaction: 2CO+2NO→N₂+2CO₂, whereby theconcentration of emitted nitrogen oxides is decreased.

At this time, in the description of the reaction formulas, the reasonwhy NO is used instead of NOx is that the composition of generatednitrogen oxides at a high temperature can be described approximatelybecause NO is a main component and the amount of NO₂ is merely several%. Even if NO₂ is present, it is considered to be reduced by CO in asimilar manner to that of NO.

Both the burner and the water tube group adjust the concentration ratio,that is, the concentration ratio is adjusted using the concentrationratio characteristics of the burner and the group of water groups. Theconcentration ratio characteristics refer to the characteristics inwhich the concentrations of carbon monoxide and nitrogen oxides afterthe passage through the whole or a part of the water tube group, whichare generated on combustion with an air ratio of the burner beingchanged, change. Further, the concentration ratio characteristics arebasically determined by the concentration ratio characteristics by theburner, and the water tube group typically have a function of changing apart of the concentration ratio characteristics of the burner or holdingthe concentration ratio characteristics thereof. The form of the watertube group includes a first aspect (corresponding to the PatentDocuments 1 to 4) in which a water tube group are arranged in acombustion space with a combustion space being hardly provided in thevicinity of the burner, and a second aspect having a combustion spacebetween the burner and the water tube group. In the first aspect, acombustion reaction proceeds in a gap of the water tube group. In thecase where the water tube group is set to be the first aspect, theconcentration of carbon monoxide is increased and the concentration ofnitrogen oxides is suppressed by cooling of gas during the combustionreaction. In the case where the water tube group is set to be the secondaspect, typically, the concentration ratio characteristics by the burnerare kept with being hardly changed.

Then, the burner and the water tube group have the followingcharacteristics of air ratio-NOx/CO. The characteristics of airratio-NOx/CO refer to the characteristics of obtaining the concentrationratio among oxygen, nitrogen oxides, and carbon monoxide in gas on aprimary side of the catalyst at which the concentration of nitrogenoxides on a secondary side of the catalyst is decreased to substantiallyzero when the air ratio is adjusted to the set air ratio by theair-ratio adjusting device. The characteristics of air ratio-NOx/COpreferably set the concentration of the nitrogen oxides on the primaryside of the catalyst at 300 ppm or less. Thus, the use amount of thecatalyst can be decreased.

Further, the concentration ratio adjustment by the burner and the watertube group is performed by determining characteristics of airratio-NOx/CO (concentration ratio characteristics) based on experimentaldata. As a result of the concentration ratio adjustment, theconcentration of carbon monoxide in the gas on a primary side of thecatalyst becomes substantially equal to or more than the sum of theconcentration of carbon monoxide decreased in the catalyst by oxidationof carbon monoxide and the concentration of carbon monoxide decreased inthe catalyst by reduction of nitrogen oxides by carbon monoxide. Here,the present invention also includes the case where elements other than awater tube group are included as the constituent elements participatingin the concentration ratio adjustment.

In the concentration ratio adjustment, it is preferred that the airratio is controlled to a set air ratio of substantially 1.0, becauseenergy saving can be achieved. Further, the concentration ratioadjustment is performed preferably by suppressing the amount of nitrogenoxides and the amount of carbon monoxide to a predetermined amount orlower by adjusting a combustion temperature, i.e., cooing gas during acombustion reaction from the burner, and by preventing the obtainedconcentration of carbon monoxide from being decreased by keeping the gastemperature. Carbon monoxide is likely to be oxidized at a gastemperature of about 900° C. or higher. Therefore, the burner and thewater tube group are constituted preferably so that the gas temperatureon a primary side of the catalyst is kept at 600° C. or lower.

The formula representing the range of the concentration ratio can beexpressed by the formula (3). Here, the value (value of a concentrationratio) of ([NOx]+2[O₂])/[CO] is set to be 2.0 or less, and preferably1.5 or less. Further, the concentration of nitrogen oxides ([NOx]) is atotal concentration of the concentration of nitrogen monoxide ([NO]) andthe concentration of nitrogen dioxide ([NO₂]). Further, theconcentration ratio among the concentration of carbon monoxide, theconcentration of nitrogen oxides, and the concentration of oxygensatisfying the formula (3) is referred to as a predeterminedconcentration ratio.

In the case where the value of the predetermined concentration ratio is1.0, the concentrations of oxygen, nitrogen oxides, and carbon monoxideemitted from the catalyst can be set to be zero theoretically. However,it is understood that a slight amount of carbon monoxide is emittedbased on an experiment. ([NOx]+2[O₂])/[CO]=1 in the formula (1) istheoretically derived from the first and second reactions, consideringthe experimental results.

Here, how to derive ([NOx]+2[O₂])/[CO]=1 will be described. This formulatypically satisfies the predetermined reference concentration ratio K0,so it is referred to as a predetermined reference concentrationsatisfying formula.

In the catalyst, it is known that the first reaction (I) is effected asa main reaction.

CO+½O₂→CO₂  (I)

Further, in a catalyst using a noble metal catalyst such as Pt, a NOreduction reaction by CO is effected by the second reaction (II) in anatmosphere free of oxygen.

CO+NO→CO₂+½N₂  (II)

Then, paying attention to the concentration of substances contributingto the reactions such as the first reaction (I) and the second reaction(II), the reference concentration satisfying formula is derived.

More specifically, assuming that a CO concentration, an NOconcentration, and an O₂ concentration are respectively [CO] ppm, [NO]ppm, and [O₂] ppm, the concentration of oxygen which can be removed byCO by the formula (1) is expressed by the following formula (III).

2[O₂]=[CO]_(a)  (III)

Further, in order to effect the reaction of the formula (II), it isnecessary that CO is equivalent to NO, and the relationship of thefollowing formula (IV) is held.

[CO]_(b)=[NO]  (IV)

In the case where the reactions of the formulae (I) and (II) areeffected continuously in the catalyst, a concentration relationship ofthe following formula (V) obtained by adding the formula (III) to theformula (IV) is required.

[CO]_(a)+[CO]_(b)=2[O₂]+[NO]  (V)

[CO]_(a)+[CO]_(b) is the same components, so the CO concentration in gason a secondary side of the catalyst can be expressed by [CO].

Accordingly, the predetermined reference concentration ratio satisfyingformula, i.e., a relationship: [CO]=2[O₂]+[NO] can be derived.

In the case where the value of the predetermined concentration ratio issmaller than 1.0, carbon monoxide is present in a concentration morethan necessary for reducing the nitrogen oxides, so the concentration ofemitted oxygen is zero and carbon monoxide remains in gas after thepassage through the catalyst. Therefore, a lower value of theconcentration ratio in the formula (3) is not provided. In the casewhere carbon monoxide is contained after the passage through thecatalyst, it is preferred to further provide oxidizing device foroxidizing the remaining carbon monoxide. The oxidizing device can beconstituted by providing a catalyst separately from the catalyst, andoxygen is injected on an upstream side of the catalyst to oxidize carbonmonoxide.

Further, the value of the concentration ratio of 2.0 exceeds 1.0, whichis obtained experimentally. The reason for this is considered asfollows. The reaction effected in the catalyst has not been clarifiedcompletely, and it is considered that an auxiliary reaction is effectedother than the main reaction such as the first reaction and the secondreaction. As one auxiliary reaction, a reaction in which hydrogen isgenerated by the reaction between vapor and carbon monoxide, andnitrogen oxides and oxygen are reduced by the hydrogen is considered.

Next, the constituent elements of embodiment modes of the presentinvention will be described further. The burner is assumed preferably tobe a primary aerated-type premixed burner that subjects a gas fuel topremixed combustion. In order to allow the first reaction and the secondreaction to be effected effectively in the catalyst, the concentrationratio as represented by the formulae (2) and (3) regarding oxygen,nitrogen oxides, and carbon monoxide is important. By using a premixedburner as the burner, the predetermined concentration ratio can beobtained relatively easily in a low air ratio region.

Further, assuming that the oxygen concentration O₂ on a primary side ofthe catalyst is 0%<O₂≦1.00% under the condition satisfying the formula(3), the air ratio becomes substantially 1.0, and energy saving isrealized in addition to low NOx and low CO in which emissionconcentration is close to zero, whereby an energy-saving combustionapparatus with a low pollution can be provided.

Further, the water tube group are used as water tubes in the case wherethe combustion apparatus is a boiler, and absorbing liquid concentrationtubes in the case where the combustion apparatus is a reproducer. Then,the water tube group also have a function of controlling the temperatureof gas flowing to the catalyst to be in the vicinity of an activationtemperature of the catalyst. More specifically, the gas temperature iscontrolled to be the one at which durability is taken into considerationby allowing the first reaction and the second reaction to be effectedeffectively and suppressing the degradation by the temperature. Althoughthe water tube group are a plurality of water tubes for heat exchange ofthe gas from the burner, a plurality of water tubes can be constitutedby meandering one water tube as in a water tube of a gas water heater.

The catalyst has a function of reducing the nitrogen oxides efficientlyin a state where the gas does not contain HC. The catalyst is configuredso as to be provided in a wake flow or at a midpoint of the water tubegroup, and carry a catalyst activating substance on a matrix having airpermeability, and the structure of the catalyst is not particularlylimited. As the matrix, metal such as stainless steel and ceramic areused, and subjected to a surface treatment of enlarging the contact areawith respect to exhaust gas. As the catalyst activating substance,platinum is generally used, and noble metal (Ag, Au, Rh, Ru, Pt, Pd)typified by platinum or a metal oxide can be used depending onimplementation. In the case where the catalyst is provided at a midpointof the water tube group, the catalyst can be provided in a gap among aplurality of water tubes, and the catalyst can be configured so as tocarry a catalyst activating substance on the surface by using the watertube as a matrix.

The air-ratio adjusting device includes flow rate adjusting device, amotor for driving the flow rate adjusting device, and control device forcontrolling the motor. The flow rate adjusting device changes either oneor both of the combustible air amount and the fuel amount of the burner,thereby adjusting the air ratio of the burner. In the case where theflow rate adjusting device adjusts the combustible air amount, the flowrate adjusting device is preferably a damper (including a valve). As thestructure of the damper, a rotation type that changes the aperture of aflow path with a valve body rotating with respect to a rotation axis,and a slide type can be used, which slides with respect to thecross-sectional opening of a flow path to change the aperture of theflow path.

In the case where the flow rate adjusting device changes a combustibleair amount, preferably, the flow rate adjusting device is provided withan air flow path between a blower and fuel supply device, and can alsobe provided on a suction port side of the blower such as the suctionport of the blower.

The motor is preferably device for driving the flow rate adjustingdevice, which is capable of controlling the opening amount of the flowrate adjusting device in accordance with the driving amount and iscapable of adjusting the driving amount per unit time. The motorconstitutes a part of “mechanical control device” for controlling theair ratio stably. “Being capable of controlling the opening amount inaccordance with a driving amount” refers to the capability of stopcontrolling the aperture of the flow rate adjustment valve at aparticular position if the driving amount is determined. Further, “beingcapable of adjusting the driving amount per unit time” refers to thecapability of adjusting the responsiveness of the position control.

As the motor, preferably, a stepping motor (which can also be called astep motor) can be used, and a gear motor (which can also be called ageared motor), a servo motor, or the like can be used. In the case ofthe stepping motor, the driving amount is determined based on a drivingpulse, and the aperture position of the flow rate adjusting device ismoved by opening/closing by the amount in accordance with the number ofdriving pulses from the reference aperture position, thereby beingcontrolled to be any intended stop position. Further, in the case of thegear motor or the servo motor, the driving amount is a opening/closingdriving time, and the aperture position of the flow rate adjustingdevice is moved by opening/closing by the amount in accordance with theopening/closing driving time from the reference aperture position,thereby being controlled to be any intended stop position.

As the sensor, an oxygen concentration meter can be suitably used, whichrepresents an excess oxygen concentration in an oxygen excess region,and represents, as a negative value, an insufficient oxygenconcentration required for burning unburned gas such as carbon monoxideat an air ratio of m=1.0 in a fuel excess region.

Further, as the sensor, an oxygen concentration sensor and a carbonmonoxide concentration sensor can be combined to obtain an air ratioapproximately.

Although the sensor is preferably attached on a secondary side of thecatalyst, the attachment position is not limited thereto, and the sensorcan be attached on a primary side of the catalyst or on a downstreamside in the case where an exhaust heat collector is provided on adownstream side of the catalyst.

The air-ratio adjusting device inputs a detected value of the sensorbased on a previously stored air ratio control program and controls thedriving amount of the motor by feedback, and controls the air ratio tobe a set air ratio of 1 so that the concentration of carbon monoxide inthe gas on a primary side of the catalyst is substantially equal to ormore than a value obtained by adding the concentration of carbonmonoxide decreased in the catalyst by the oxidation to the concentrationof carbon monoxide decreased in the catalyst by the reduction, or theformula (3) is satisfied.

The air ratio control program is preferably configured by providing afirst control band for changing the driving amount (which can beexpressed as a time per one driving unit) per unit time of the motor inaccordance with the difference between the detected air ratio and theset air ratio and a second control band for setting the driving amountper unit time to be a fixed predetermined value on an outside of thefirst control band, thereby controlling the driving amount of the motor.This control constitutes the electrical control device for controllingthe detected air ratio to be in a range which is set with the set airratio as a center. Note that the air ratio control program is notlimited to this control system, and can perform various PID controls.The control amount in the first control band can be controlled by aformula of a product of the difference between the detected air ratioand the set air ratio, and a set gain. Due to such a control, the setair ratio can be controlled rapidly, and an effect capable ofcontrolling with less overshoot and hunting can be exhibited.

The concentration ratio adjustment by the burner and the water tubegroup includes a form conducted by elements constituting a gas passagefrom the burner to the catalyst other than the water tube group andelements included in the gas passage.

Further, in the mechanical control device, a combustible air supplypassage can be constituted by a main passage and an auxiliary passage inparallel to the main passage. The device can be constituted to roughlyadjust the air flow rate by the operation of the valve body provided inthe main passage, and to minutely adjust the air flow rate by theoperation of the valve body provided in the auxiliary passage. Further,in the mechanical control device, fuel supply passage can be constitutedby a main passage and an auxiliary passage in parallel to the mainpassage. The device can be constituted to roughly adjust the air flowrate by the operation of the valve body provided in the main passage,and to minutely adjust the air flow rate by the operation of the valvebody provided in the auxiliary passage.

The flow rate adjusting device of the air-ratio adjusting device cancontrol the motor of the blower with an inverter. As the inverter, aninverter with a well-known configuration can be used. Even in the caseof using the inverter, the air ratio can be controlled with the airratio control program used in damper control.

EMBODIMENT MODE 5

The present invention is not limited to Embodiment Modes 1 to 5 of theboiler, and includes Embodiment Modes 5 to 7 as described below.

Embodiment Mode 5 is a boiler including: a premixed burner for burning ahydrocarbon-containing fuel to generate gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from gas generated by the premixed burner; anoxidation catalyst for oxidizing the carbon monoxide contained in thegas after the passage through the water tube group by oxygen andreducing the nitrogen oxides contained in the gas by carbon monoxide; asensor for detecting an air ratio of the premixed burner; and air-ratioadjusting device for controlling the premixed burner to a set air ratiobased on a detected signal of the sensor, in which the premixed burnerand the water tube group have characteristics of air ratio-NOx/CO inwhich concentrations of nitrogen oxides and oxygen on a secondary sideof the oxidation catalyst are set to be substantially zero to decrease aconcentration of carbon monoxide to substantially zero or apredetermined value or less, when the air ratio is adjusted to the setair ratio by the air-ratio adjusting device.

Embodiment Mode 6 is a boiler including: a premixed burner for burning ahydrocarbon-containing fuel to generate gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from gas generated by the premixed burner; anoxidation catalyst for oxidizing the carbon monoxide contained in thegas after the passage through the water tube group by oxygen andreducing the nitrogen oxides contained in the gas by carbon monoxide; asensor for detecting an air ratio of the premixed burner; and air-ratioadjusting device for controlling the premixed burner to a set air ratiobased on a detected signal of the sensor, in which the premixed burnerand the water tube group are configured so that a concentration ofcarbon monoxide in the gas on a primary side of the oxidation catalystis substantially equal to or more than a value obtained by adding aconcentration of carbon monoxide decreased in the oxidation catalyst bythe oxidation to a concentration of carbon monoxide decreased in theoxidation catalyst by the reduction, when the air ratio is adjusted tothe set air ratio by the air-ratio adjusting device.

Embodiment Mode 7 is a boiler including: a premixed burner for burning ahydrocarbon-containing fuel to generate gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from gas generated by the premixed burner; anoxidation catalyst for oxidizing the carbon monoxide contained in thegas after the passage through the water tube group by oxygen andreducing the nitrogen oxides contained in the gas by carbon monoxide; asensor for detecting an air ratio of the premixed burner; and air-ratioadjusting device for controlling the premixed burner to a set air ratiobased on a detected signal of the sensor, in which the premixed burnerand the water tube group are configured so that a concentration ratio ofthe gas before flowing to the oxidation catalyst satisfies the followingformula (3), when the air ratio is adjusted to the set air ratio by theair-ratio adjusting device:

([NOx]+2[O₂])/[CO]≦2.0  (3)

(in formula (3), [CO], [NOx], and [O₂] respectively represent aconcentration of carbon monoxide, a concentration of nitrogen oxides,and a concentration of oxygen, and a condition of [O₂]>0 is satisfied.)

The boilers in Embodiment Modes 5 to 7 exhibit the actions and effectssimilar to those in Embodiment 4. This invention includes Embodiment 8as described below.

EMBODIMENT MODE 8

Embodiment Mode 8 includes: a premixed burner for burning ahydrocarbon-containing fuel to generate gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from gas generated by the premixed burner; anoxidation catalyst for oxidizing the carbon monoxide contained in thegas after the passage through the water tube group by oxygen andreducing the nitrogen oxides contained in the gas by carbon monoxide;and air-ratio adjusting device for adjusting an air ratio of thepremixed burner. The premixed burner and the water tube group areconfigured so as to have characteristics of air ratio-NOx/CO on aprimary side of the oxidation catalyst regarding the gas containingoxygen, nitrogen oxides, and carbon monoxide on a primary side of theoxidation catalyst obtained by adjusting an air ratio in the vicinity of1.0 by the air-ratio adjusting device, and the oxidation catalyst isconfigured so as to have characteristics of air ratio-NOx/CO (secondarycharacteristics) on a secondary side of the oxidation catalyst obtainedby bringing the gas having characteristics of air ratio-NOx/CO (primarycharacteristics) on the primary side into contact with the oxidationcatalyst. The air-ratio adjusting device is set to be a value which isthe set air ratio in the NOx/CO decreasing region of the characteristicsof air ratio-NOx/CO on the secondary side, and at which a concentrationof nitrogen oxides (concentration of emitted NOx) in the secondarycharacteristics is decreased to be substantially zero. The concentrationof nitrogen oxides can be decreased to be substantially zero preferablyby controlling the air ratio of the premixed burner to be substantially1.0. This control is performed preferably by an air ratio on a secondaryside of the oxidation catalyst, and can also be performed by theconcentration of O₂ on a primary side so that the concentration ofoxygen (O₂ concentration) on a primary side of the catalyst capable ofsatisfying the set air ratio of substantially 1.0 becomes apredetermined concentration as a result of the reaction in the catalyst.The primary characteristics are concentration ratio characteristics bythe burner and the water tube group of the present invention, andinclude characteristics of air ratio-NOx and characteristics of airratio-CO. Further, the secondary characteristics are the characteristics(catalyst characteristics) by the catalyst, and include characteristicsof air ratio-NOx and characteristics of air ratio-CO.

In Embodiment Mode 8, the gas generated on combustion by the burner issubjected to a heat absorbing function in the water tube group to becomegas containing oxygen, nitrogen oxides, and carbon monoxide at apredetermined concentration ratio. Then, by changing the air ratio ofthe burner in a low air ratio region, the primary characteristics thatare concentration ratio characteristics by the burner and the water tubegroup and the secondary characteristics by the characteristics of thecatalyst are obtained.

EMBODIMENT MODE 9

Embodiment Mode 8 can be expressed by Embodiment Mode 9 as describedbelow of a boiler. Embodiment Mode 9 includes: a premixed burner forburning a hydrocarbon-containing fuel to generate gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from gas generated by the premixedburner; a catalyst that is brought into contact with the gas after thepassage through the water tube group, and decreases carbon monoxide anddoes not decrease nitrogen oxides when a concentration ratio of oxygen,nitrogen oxides, and carbon monoxide in gas is in a NOx non-decreasingregion and decreases carbon monoxide and nitrogen oxides when theconcentration ratio is in a NOx decreasing region; and air-ratioadjusting device for adjusting the ratio of an air amount and/or a fuelamount supplied to the premixed burner. The air-ratio adjusting deviceadjusts the ratio of an air amount and/or a fuel amount so that theconcentration ratio becomes the NOx decreasing region, and theadjustment is configured so that the concentration of nitrogen oxides ona secondary side of the oxidation catalyst becomes substantially zero.The adjustment of concentration ratio of Embodiment Mode 9 includesAdjustment 0 and Adjustment 1. Further, in the adjustment of theconcentration ratio, preferably, the concentration of oxygen on asecondary side of the oxidation catalyst is decreased to besubstantially zero. Carbon monoxide in the catalyst is decreased byoxidation, and nitrogen oxides are decreased by reduction by carbonmonoxide.

Further, in Embodiment Mode 9, preferably, the concentration ratioadjustment by the premixed burner and the water tube group suppresses aconcentration of hazardous substances to be generated to a setconcentration or less. Here, the hazardous substances (which can also becalled a pollutant) are nitrogen oxides and carbon monoxide. The setconcentration can be set to be, for example, 300 ppm, in the case wherethe hazardous substances are nitrogen oxides. More specifically, theconcentration of hazardous substances to be generated is suppressed to aset concentration or less by the concentration ratio adjustment, wherebythe treatment amount in the oxidation catalyst, i.e., the amount of thecatalyst can be decreased. A device for suppressing the gas temperatureand suppressing the concentration of nitrogen oxides to be apredetermined value or less is referred to as nitrogen oxide generationsuppressing device. The nitrogen oxide generation suppressing deviceincludes a device for cooling gas in a combustion reaction by a watertube group, device for re-circulating exhaust gas, device for addingwater or vapor to gas in the combustion reaction, and a combination of aplurality of these devices. The nitrogen oxide generation suppressingdevice is applicable not only to Embodiment Mode 9 but also to any otherembodiment modes.

EMBODIMENT MODE 10

Further, the present invention includes Embodiment Mode 10 describedbelow of a boiler. Embodiment Mode 10 is a boiler including: a premixedburner for burning a hydrocarbon-containing fuel to generate gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from gas generated by the premixedburner; a catalyst that performs, as main reactions, a first reaction ofbeing brought into contact with gas containing oxygen, nitrogen oxides,and carbon monoxide after the passage through the water tube group, andoxidizing carbon monoxide by oxygen in the gas, and a second reaction ofreducing nitrogen oxides by carbon monoxide in the gas; and controldevice (concentration ratio adjusting device) for adjusting the ratiobetween combustible air and a fuel of the premixed burner. The catalysthas the following characteristics. Assuming that a concentration ratioamong oxygen, nitrogen oxides, and carbon monoxide in gas on a primaryside of the catalyst at which concentrations of nitrogen oxides andcarbon monoxide on a secondary side becomes substantially zero is apredetermined reference concentration ratio, when the concentrationratio is the predetermined reference concentration ratio, theconcentrations of nitrogen oxides and carbon monoxide on the secondaryside of the catalyst are set to be substantially zero, and when theconcentration of oxygen on the primary side is set to be higher than thereference oxygen concentration corresponding to the predeterminedreference concentration ratio, oxygen in a concentration in accordancewith the difference between the concentration of oxygen on the primaryside and the reference oxygen concentration is detected on the secondaryside of the catalyst, and the concentration of carbon monoxide on thesecondary side of the catalyst is set to be substantially zero and theconcentration of nitrogen oxides is decreased. Further, when theconcentration of oxygen on the primary side is set to be lower than thereference oxygen concentration, carbon monoxide at a concentration inaccordance with the difference between the concentration of oxygen onthe primary side and the reference oxygen concentration is detected onthe secondary side of the catalyst, and the concentration of nitrogenoxides on the secondary side of the catalyst is set to be zero and theconcentration of carbon monoxide is decreased. The concentration ratioadjusting device is characterized in that it adjusts the ratio betweenthe combustible air amount and the fuel amount of the premixed burnerbased on the concentration of oxygen on the secondary side of thecatalyst, using the characteristics of the catalyst, thereby adjustingthe concentration of oxygen on the primary side of the catalyst withrespect to the reference oxygen concentration, and decreasing theconcentrations of nitrogen oxides and carbon monoxide on the secondaryside of the catalyst. This adjustment includes Adjustment 0 andAdjustment 1.

Embodiment Modes 8 and 9 are expressed based the primary characteristicsand the secondary characteristics of the premixed burner and water tubegroup with respect to the air ratio obtained by the concentration ofoxygen and/or the concentration of carbon monoxide, etc. on thesecondary side of the catalyst. In contrast, Embodiment Mode 10 isexpressed based on the primary characteristics and the catalystcharacteristics of the premixed burner and the water tube group withrespect to the concentration of oxygen on the primary side of thecatalyst.

The catalyst characteristics are the following characteristics. Morespecifically, as shown in a schematic view of FIG. 7, the catalystcharacteristics have a characteristic line L (secondary side [NOx]=0,[CO]=0 line) of the concentration ratio on the primary side of thecatalyst. When the concentration ratio K on the primary side of thecatalyst is positioned on the line L, the concentration of nitrogenoxides and the concentration of carbon monoxide on the secondary side ofthe catalyst become substantially zero. In the line L, the predeterminedconcentration ratio K in the formula (3) corresponds to 1.0 (K0=1.0 inthe formula (2)) theoretically. However, as described above, since ithas been experimentally confirmed that the concentrations of nitrogenoxides and carbon monoxide on the secondary side of the catalyst can beset to be substantially zero in a range of the concentration ratio Kexceeding 1.0 up to 2.0, the characteristic line L on the secondary sideis not limited to the line in FIG. 7.

Then, a concentration ratio K among oxygen, nitrogen oxides, and carbonmonoxide at an intersection between a line M of the primarycharacteristics of the burner and the water tube group and thecharacteristic line L on the secondary side is assumed to be apredetermined specific reference concentration ratio K0X (hereinafter,referred to as a specific reference concentration ratio). When theconcentration ratio K on the primary side of the catalyst is adjusted tothe specific reference concentration ratio K0X (Adjustment 0), theconcentrations of nitrogen oxides and carbon monoxide on the secondaryside of the catalyst are set to be substantially zero. Then, when theconcentration of oxygen on the primary side is set to be higher than areference oxygen concentration SK corresponding to the specificreference concentration ratio K0X, i.e., the concentration of oxygen onthe primary side is set to be higher by the air-ratio adjusting device,oxygen at a concentration in accordance with the difference between theconcentration of oxygen on the primary side and the reference oxygenconcentration is detected on the secondary side of the catalyst, theconcentration of nitrogen oxides on the secondary side of the catalystbecomes lower than the concentration of nitrogen oxides on the primaryside, and the concentration of carbon monoxide on the secondary sidebecomes substantially zero. Further, when the concentration of oxygen ona primary side is set to be lower than the specific referenceconcentration ratio K0X (Adjustment 1), carbon monoxide at aconcentration in accordance with the difference between theconcentration of oxygen on the primary side and the reference oxygenconcentration is detected on the secondary side of the catalyst, theconcentration of nitrogen oxides on the secondary side of the catalystbecomes substantially zero, and the concentration of carbon monoxide onthe secondary side is decreased.

If those characteristics of the catalyst and the primary characteristicsof the premixed burner and the water tube group are used, theconcentration of emitted NOx and the concentration of emitted CO can beeasily controlled to be substantially zero by controlling theconcentration of oxygen and/or the concentration of carbon monoxide onthe secondary side of the catalyst to be zero, i.e., by controlling anair ratio to be zero. More specifically, by controlling theconcentration of oxygen and/or the concentration of carbon monoxide onthe secondary side of the catalyst, energy saving by burning at an airratio of 1.0 and ultra-low pollution in which the concentration ofemitted NOx and the concentration of emitted CO are substantially zerocan be realized simultaneously.

Further, by controlling the concentration of oxygen and/or theconcentration of carbon monoxide on the secondary aide of the catalystin the vicinity of zero, the concentration of emitted NOx can bedecreased to a value close to zero although it cannot be decreased to besubstantially zero.

In Embodiment Modes 1 to 9 as described above, preferably, a feed-waterpreheater is provided as an exhaust heat recovery unit on the secondaryside (wake flow side) of the catalyst, whereby a great amount of heatgenerated by oxidation of carbon monoxide in the catalyst can berecovered for supplying water. Further, the catalyst is preferablyprovided in a gas duct of a wake flow of the water tube group so as toavoid the contamination due to the drop of condensed water from theexhaust heat recovery unit, and the catalyst is prevented from beingcontaminated by condensed water dropping from the exhaust heat recoveryunit.

Further, Embodiment Modes 1 to 9 as described above preferably include asecond sensor (second sensor) for detecting abnormality of the catalystor the sensor (first sensor), notification device, and control devicefor detecting the abnormality based on the detected value of the secondsensor, and executing an abnormality time control program for notifyingthe abnormality by the notification device.

In Embodiment Modes 1 to 9 as described above, when the catalyst losesthe original function due to the degradation or the like, a set value ormore of carbon monoxide is contained in gas on the secondary side of thecatalyst. When the detected value of the second sensor exhibits anabnormality value, the control device determines that the catalyst orthe first sensor is abnormal, and notifies the abnormality by thenotification device. Due to the notification of abnormality, anadministrator and a maintenance operator of a boiler can check thecatalyst or the first sensor to perform maintenance such as exchange ofthe catalyst.

The second sensor detects the abnormality of the catalyst or the firstsensor. In the case where the second sensor detects the abnormality ofthe catalyst, the second sensor is preferably a CO sensor for detectingthe increase in a concentration of carbon monoxide on the secondary sideof the catalyst. However, the second sensor can be a NOx sensor fordetecting the increase in a concentration of nitrogen oxides on thesecondary side of the catalyst. Further, two sensors, a CO sensor and aNOx sensor, can constitute the second sensor.

In the case where the second sensor detects the abnormality of the firstsensor, the second sensor is set to be a sensor for detecting the sameair ratio as that of the first sensor.

The notification device is not particularly limited, as long as itnotifies abnormality of the catalyst visually and/or acoustically.Further, the notification device can be configured so as to be attachedto a controller constituting the control device or so as to notifyabnormality via a communication line at a position away from thecombustion apparatus.

The abnormality time control program includes a program for controllingthe air ratio to be a second set air ratio λ2 (for example, 1.25) sothat the concentration of carbon monoxide on the primary side of thecatalyst becomes a second set value equal to or lower than an emissionreference value, in order to set the concentration of carbon monoxide onthe secondary side of the catalyst to be an emission reference value orless even in the case where the catalyst or the first sensor does notfunction normally.

The abnormality time control program can be configured as follows. Inthe case where the second sensor is a sensor for detecting theabnormality of the catalyst, the control device drives the notificationdevice to notify abnormality, and shifts the combustion at the first setair ratio λ1 to the combustion at the second set air ratio λ2, when thedetected value of the second sensor exceeds the set value that is areference value for determining abnormality. The combustion at thesecond set air ratio λ2 is a hasty operation. However, the concentrationof carbon monoxide in gas on the primary side of the catalyst is 300 ppmor less that is an emission reference value, so the concentration ofemitted carbon monoxide can be set to be an emission reference value orless even if the catalyst does not function at all.

Further, in the case where the second sensor is a sensor for detectingthe abnormality of the first sensor, when the second sensor has a normalvalue, i.e., detects a set air ratio, whereas the first sensor has anabnormal value, i.e., does not detect the set air ratio, the controldevice changes the set air ratio. Then, when the first sensor does notdetect an air ratio changed by the first sensor, and the second sensordetects the air ratio changed by the second sensor, the control devicedetermines the first sensor to be abnormal, and switches to the airratio control by the second sensor.

The administrator or the maintenance member of the combustion apparatusperforms an exchange and maintenance of the catalyst based on thenotification by the notification device, thereby normalizing thefunction of the catalyst. After that, the combustion at the first setair ratio λ1 is started.

In the above-mentioned embodiment modes, the flow rate adjusting deviceof the air-ratio adjusting device preferably includes abnormalitydetection device for detecting the abnormality of itself. When thesecond sensor detects abnormality, the catalyst, the first sensor, theflow rate adjusting device, and the like can be considered as the causeof the abnormality. The cause of abnormality can be specified easily byproviding the abnormality detection device.

EMBODIMENT 1

Next, an explanation will be made by referring to the drawings for anembodiment in which the combustion apparatus of the present invention isapplied to a steam boiler: FIG. 1 is a longitudinal sectional view forexplaining a steam boiler of Embodiment 1; FIG. 2 is a sectional viewtaken along line II to II in FIG. 1; FIG. 3 is a drawing showing aconstitution of major parts when an oxidation catalyst in FIG. 2 isviewed from a direction in which exhaust gas flows; FIG. 4 is a drawingshowing the characteristics of air ratio-NOx/CO in Embodiment 1; FIG. 5is a partial sectional view for explaining a damper position adjustingdevice of Embodiment 1, which is in operation; FIG. 6 is a partialsectional view for explaining the damper position adjusting device inoperation; FIG. 7 is a pattern diagram for explaining thecharacteristics of a burner and endothermic device and thecharacteristics of a catalyst in Embodiment 1; FIG. 8 is a drawing forexplaining the output characteristics of the sensor of Embodiment 1;FIG. 9 is a drawing for explaining the motor control characteristics inEmbodiment 1; and FIG. 10 is a drawing for explaining the NOx and COdecreasing characteristics of Embodiment 1.

At first, an explanation will be made for the steam boiler ofEmbodiment 1. The steam boiler is provided with a burner 1, a storagewater heater body 3 including a heat transfer tube (water tube) group 2as endothermic device for absorbing the heat of gas generated from theburner 1, an oxidation catalyst (hereinafter sometimes simply referredto as “catalyst”) 4 through which gas containing each of oxygen,nitrogen oxides, and carbon monoxide at the predetermined concentrationratios after passing through the heat transfer tube group 2 in contacttherewith, thus oxidizing carbon monoxide and also reducing nitrogenoxides, fuel supply device for supplying fuel gas to the burner 1,combustible air supply device 6 for supplying combustible air to theburner 1 to premix fuel with the combustible air, a sensor 7 fordetecting the concentration of oxygen downstream from the catalyst 4,and a controller 8 as a boiler controller for inputting signals such asthose from the sensor 7 or others to control the fuel supply device 5,the combustible air supply device 6, and others.

The burner 1 is a complete premix-type burner having a flat combustionface (face of ejecting premixed air). The burner 1 is similar inconstitution to the burner described in Patent Document 1.

The storage water heater body 3 is provided with an upper header 9 and alower header 10 to arrange a plurality of inner water tubes 11, 11 . . ., which constitute the water tube group 2 between the headers. Then, asshown in FIG. 2, a pair of water tube walls 14, 14 constituted byconnecting outer water tubes 12, 12 . . . by using connection members13, 13 . . . are provided on both ends of the storage water heater body3 in a longitudinal direction, thereby forming a first gas duct 15through which gas from the burner 1 passes substantially linearlybetween these water tube walls 14, 14, the upper header 9 and the lowerheader 10. The burner 1 is installed on one end of the first gas duct15, and a second gas duct (smoke duct) 17 through which exhaust gaspasses is connected to the other end thereof, which is an exhaust gasoutlet 16. The burner 1 and the storage water heater body 3 used inEmbodiment 1 are known.

The second gas duct 17 includes a horizontal part 18 and a perpendicularpart 19, and the catalyst 4 is loaded at the horizontal part 18. Afeed-water preheater 20, as an exhaust heat recovery system, is attachedto the perpendicular part 19 so as to be positioned downstream from thecatalyst 4, and the sensor 7 is placed between the catalyst 4 and thefeed-water preheater 20.

The burner 1 and constituents from the burner 1 including the water tubegroup 2 to the catalyst 4 (in particular, the burner 1 and the watertube group 2 are major parts) are provided with functions to adjust theconcentration ratio K in gas on the primary side of the catalyst 4 tothe predetermined concentration ratios K0 and K1. In other words, thoseconstituents are structured so that there are provided thecharacteristics of air ratio-NOx/CO as shown in FIG. 4 when adjustmentis made to a set air ratio by air-ratio adjusting device 28 to bedescribed later. The characteristics of air ratio-NOx/CO arecharacteristics of air ratio-NOx/CO on the primary side of the catalyst4, which are obtained when the air-ratio adjusting device 28 iscontrolled to conduct combustion at a varied air ratio, (hereinafter,referred to as primary characteristics). Then, the catalyst 4 hascharacteristics of air ratio-NOx/CO on the secondary side of thecatalyst 4, which are obtained by allowing the gas having the primarycharacteristics to be in contact with the catalyst 4, (hereinafter,referred to as secondary characteristics). The primary characteristicsare the characteristics of constituents from the burner 1 to thecatalyst 4, whereas the secondary characteristics are the concentrationratio characteristics of the catalyst 4. The primary characteristics areto decrease the concentration of NOx and that of carbon monoxide on thesecondary side of the catalyst 4 to substantially zero when the airratio is adjusted to 1.0. In this instance, the predetermined referenceconcentration ratio K0 in gas on the primary side of the catalyst 4 isgiven as a specific reference concentration ratio (hereinafter, referredto as a specific reference concentration ratio) K0X (refer to FIG. 7).

FIG. 4 is a pattern diagram in which the low air ratio region Z2 givenin FIG. 17 is elongated, although the vertical axis and the lateral axisare differently scaled. In FIG. 4, a first line (characteristic line) Eindicates the concentration of CO on the primary side of the catalyst 4,and a second line F indicates the concentration of NOx on the primaryside. Further, a third line J indicates the concentration of CO on thesecondary side of the catalyst 4, having such characteristics that theconcentration of CO is decreased to substantially zero at an air ratio1.0 or more and the concentration is abruptly increased as the air ratiois lower than 1.0. Still further, a fourth line U indicates theconcentration of NOx on the secondary side of the catalyst 4, havingsuch characteristics that the concentration of NOx is decreased tosubstantially zero in a predetermined region having the air ratio of 1.0or lower, and the concentration is increased substantially from zero,when the air ratio is in excess of 1.0 and soon equal to theconcentration on the primary side of the catalyst 4. A region equal toor lower than an air ratio at which the concentration of NOx on thesecondary side of the catalyst 4 is equal to the concentration on theprimary side is referred to as a NOx/CO decreasing region. A lower limitof the NOx/CO decreasing region is given as an air ratio at which theconcentration of CO on the secondary side of the catalyst 4 is 300 ppm(CO exhaust standards in Japan). Characteristics of air ratio-NOx/CO ofthe low air ratio region are new characteristics, which have not yetbeen subjected to research.

The catalyst 4 is provided with functions of oxidizing carbon monoxidecontained in the gas free of hydrocarbons after passing through thewater tube group 2 (first reaction) and also reducing nitrogen oxides(second reaction). In Embodiment 1, used is a catalyst in which acatalyst activating substance is platinum. As already having beenexplained in the section of “Best Mode for carrying out the Invention,”when theoretical consideration is given on the basis of experimentalresults, there may be a first reaction in which the gas satisfyingformula (3) of the concentration ratio is in contact with the catalystactivating substance of the catalyst 4 to oxidize mainly carbon monoxideand a second reaction in which nitrogen oxides are reduced by carbonmonoxide. Whether the first reaction proceeds or not will be determineddepending on the concentration of oxygen. In the catalyst 4, it isconsidered that the first reaction is predominant over the secondreaction.

The catalyst 4 will be more specifically explained. The catalyst has astructure as shown in FIG. 3 and is formed as follows, for example. Manyfine irregularities are formed on the respective surfaces of a flatplate 21 and a corrugated plate 22, both of which are made of stainlesssteel, as the matrix, thereby holding a catalyst activating substance(not illustrated) on the surfaces. Then, the flat plate 21 having apredetermined width is placed on the corrugated plate 22, which are thenwound helically and formed into a roll shape. A side plate 23 is used toenclose and fix the thus roll-shaped substance to form the catalyst.Platinum is used as the catalyst activating substance. In addition, FIG.3 shows the flat plate 21 and the corrugated plate 22 only partially.

The catalyst 4 is active in oxidation in a low temperature region andplaced at the horizontal part 18, which is on its way to the second gasduct 17, that is, at a position where exhaust gas temperature isapproximately in a range of 100° C. to 350° C., and preferably 150° C.to 350° C. Then, the catalyst 4 is removably attached to the second gasduct 17 so as to be exchanged when deteriorated in performance.

The fuel supply device 5 is constituted so as to include a fuel gassupply tube 24 and a flow rate adjusting valve 25 installed on the fuelgas supply tube 24 to adjust a fuel flow rate. The flow rate adjustingvalve 25 is provided with functions of controlling fuel supply at a highcombustion flow rate and a low combustion flow rate.

The combustible air supply device 6 is constituted so as to include ablower 26, an air supply duct 27 for supplying combustible air from theblower 26 to the burner 1, and air-ratio adjusting device 28 foradjusting an air ratio of the burner 1 by adjusting the amount ofcombustible air flowing through the air supply duct 27. The fuel gassupply tube 24 is connected inside the air supply duct 27 so as to ejectfuel gas.

The air-ratio adjusting device 28 is constituted so as to include adamper 29 as flow rate adjusting device for adjusting an aperture(cross-sectional area of the flow channel) of the air supply duct 27, adamper position adjusting device 30 for adjusting an aperture positionof the damper 29, and the controller 8 for controlling the operation ofthe damper position adjusting device 30.

The damper position adjusting device 30 is, as shown in FIG. 5, providedwith a driving shaft 32 removably connected to a rotating shaft 31 ofthe damper 29. The driving shaft 32 can be rotated by a motor 34 via areduction gear 33. The motor 34 includes any motor freely adjustable forrotation position and stop position. In the present embodiment, astepping motor (pulse motor) is used.

The driving shaft 32 is connected to the rotating shaft 31 of the damper29 via a coupling 35, by which it can be rotated substantially coaxiallyin an integral manner. The coupling 35 is formed in a steppedcylindrical shape, the central part of which is provided with a minordiameter hole 36 and a major diameter hole 37, which have penetratedaxially. The driving shaft 32 is inserted into the minor diameter hole36, and the driving shaft 32 is integrally fixed to the coupling 35 by afitting screw 38. The rotating shaft 31 of the damper 29 can be insertedinto the major diameter hole 37, and the rotating shaft 31 can beintegrally rotated via a key 39 together with the coupling 35.Therefore, key grooves 40 and 41 are formed on the rotating shaft 31 andthe major diameter hole 37 of the coupling 35, respectively.

The above-mentioned coupling 35 is retained in an external case 43 ofthe damper position adjusting device 30 so as to rotate freely in astate that one end thereof is inserted into the driving shaft 32, withthe other end inserted via a bearing 42. The external case 43 isconstituted in such a manner that the reduction gear 33 and the motor 34are retained on one end thereof and the coupling 35 and an abnormalrotation detecting device 44 are contained therein hermetically on theother end thereof in a state that the key groove 41-equipped majordiameter hole 37 of the coupling 35 is exposed.

The abnormal rotation detecting device 44 is provided with a plate to bedetected 45 and a detector 46. The plate to be detected 45 is extendedradially outwardly and fixed to a stepped portion at the center of thecoupling 35 in an axial direction. The plate to be detected 45 isinstalled so as to be coaxial with the coupling 35 and the driving shaft32. A slit forming region 48 having many slits 47, 47 . . . equallyspaced in a peripheral direction is installed partially at an outerperiphery of the plate to be detected 45. In the present embodiment, theslit forming region 48 is installed only in a quarter of a circular arc(90 degrees). Each of the slits 47 formed at the slit forming region 48is identical in shape and size. In the present embodiment, narrow andlong rectangular grooves along the plate to be detected 45 in the radialdirection are punched peripherally at equal intervals.

The detector 46 for detecting the slit 47 is fixed to the external case43. The detector 46 is composed of a transmission-type photo interrupterand installed in such a manner that an outer periphery of the plate tobe detected 45 is placed between a light emitting device 49 and a lightreceiving device 50. The plate to be detected 45 is placed between thelight emitting device 49 and the light receiving device 50 of thedetector 46, thereby the presence or absence of light reception from thelight emitting device 49 by the light receiving device 50 is switched bywhether or not the slit 47 on the plate to be detected 45 is arranged ata position corresponding to the detector 46 (position corresponding to alight path from the light emitting device 49 to the light receivingdevice 50). Thereby, it is possible to detect an aperture position ofthe damper 29.

The damper position adjusting device 30 is positioned so that the damper29 keeps the air supply duct 27 fully closed in a state that a slit 51at the clockwise end of the slit forming region 48 shown in FIG. 6 isplaced at a position corresponding to the detector 46 and attached tothe rotating shaft 31 of the damper 29.

Then, the slit forming region 48 is formed only at a portioncorresponding to a quarter of the plate to be detected 45, therefore, ina state that the slit 51 at the clockwise end of the slit forming region48 is placed at a position corresponding to the detector 46, the damper29 keeps the air supply duct 27 fully closed as described above. On theother hand, in a state that a slit 52 at the counter-clockwise end ofthe slit forming region 48 is arranged at a position corresponding tothe detector 46, the damper 29 keeps the air supply duct 27 fullyopened.

The damper position adjusting device 30 is constituted so that the motor34 and the detector 46 are connected to the controller 8, and being ableto control the rotation of the motor 34, while monitoring an abnormalrotation of the damper 29. More specifically, in order to control themotor 34, the damper position adjusting device 30 is provided with acircuit for preparing control signals including driving pulse to themotor 34 and able to output the thus prepared control signal to themotor 34. Thereby, the motor 34 is arbitrarily controlled for therotation angle, depending on normal rotation or reverse rotation anddriving amount, that is, the number of driving pulses. Further, themotor 34 can change the driving pulse in interval (feeding velocity),thereby making it possible to control the rotation speed.

In controlling an actual opening and closing of the damper 29, thecontroller 8 at first operates to detect an original point so that afully closed position of the damper 29 can be given as the originalpoint. First, in FIG. 5, the plate to be detected 45 is rotated in acounter-clockwise direction. On the assumption that the detector 46 isat present arranged inside the slit forming region 48 of the plate to bedetected 45, the detector 46 detects the slit 47 regularly in accordancewith the rotation of the plate to be detected 45. Therefore, thedetected pulse is output to the controller 8 as a detection signal.Then, the plate to be detected 45 is rotated until the detector 46 isplaced outside the slit forming region 48, thereby no pulse is detected.If no pulse is detected within a predetermined time, the controller 8recognizes that the detector 46 is outside the slit forming region 48,switching the rotating direction to a reverse direction. In other words,in the present embodiment, the original point is defined as a positionat which the plate to be detected 45 is rotated reversely in a clockwisedirection to detect the first pulse (slit 51 at the clockwise end)Confirmation of the original point by the clockwise rotation is made ata lower speed than the counter-clockwise rotation before the rotatingdirection is switched.

Since the thus detected original point corresponds to a fully closedposition of the damper 29, the controller 8 outputs a driving signal tothe motor 34 on the basis of this state, thus making it possible tocontrol the opening and closing of the damper 29. If the controller 8drives the motor 34 to open or close the damper 29, a detection signalof the slit 47 is obtained as a pulse from the detector 46 accordingly.Therefore, the controller 8 is able to monitor an abnormal rotation ofthe damper 29 by comparing a detection signal from the detector 46 witha control signal to the motor 34. Specifically, a control signalcomposed of driving pulse to the motor 34 is compared with a detectionsignal composed of detection pulse of the slit 47 by the detector 46,thereby monitoring the presence or absence of abnormal rotation.

For example, where no detection pulse is detected from the detector 46despite the fact that a driving pulse has been sent to the motor 34, thecontroller 8 determines it to be an abnormal rotation. In this instance,the detection pulse from the detector 46 is usually different infrequency from driving pulse to the motor 34. Therefore, control isobtained, with the difference taken into account. For example, suchcontrol is obtained that the abnormal rotation is determined only in acase where no pulse of detection signal is detected at all even afterthe elapse of a predetermined pulse of a driving signal. The controller8 performs a notification operation of the abnormal rotation and haltsthe combustion upon determination of the abnormal rotation. In contrast,the abnormal rotation can also be detected in a case where any pulse isdetected by the detector 46, despite the fact that no driving pulse hasbeen sent to the motor 34.

The controller 8 is constituted so as to control the motor 34 byreferring to a previously stored air ratio control program based onsignals detected by the sensor 7 in such a manner that an air ratio ofthe burner 1 will be a set air ratio (first control condition) and alsoa concentration ratio K of the gas on the primary side of the catalyst 4satisfies the following formula (3) at this set air ratio (secondcontrol condition).

([NOx]+2[O₂])/[CO]≦2.0  (3)

(in the formula (3), [CO], [NOx], and [O₂] represent the concentrationsof carbon monoxide, nitrogen oxides, and oxygen, respectively, andsatisfying the condition of [O₂]>0).

In Embodiment 1, it is the first control condition that gives a directcontrol. Therefore, the embodiment is constituted so that the firstcontrol condition is satisfied, by which the second control condition isautomatically satisfied. This will be explained hereinafter by referringto FIG. 4 and FIG. 7.

The characteristics of air ratio-NOx/CO given in FIG. 4 are expressedbased on the primary characteristics of constituents including theburner 1 and the water tube group 2 as well as the secondarycharacteristics of the catalyst 4. In FIG. 7, they are expressed basedon the primary characteristics of the constituents with respect to theconcentration of oxygen on the primary side of the catalyst 4 and thecharacteristics of the catalyst 4.

As shown in FIG. 7, the characteristics of the catalyst 4 are expressedby a fifth line L ([NOx] on the secondary side=0, [CO]=0 line) relatedto the predetermined reference concentration ratio K0 on the primaryside of the catalyst 4. The fifth line L is a line in which theconcentration of nitrogen oxides and that of carbon monoxide on thesecondary side of the catalyst 4 are decreased to substantially zerowhen the concentration ratio K on the primary side of the catalyst 4 ispositioned (placed) on the line, specifically, a line, which satisfiesthe predetermined reference concentration ratio K0. The fifth line Lcorresponds to a case where the predetermined concentration ratio offormula (3) is 1. In other words, the fifth line L is a line satisfyingthe following formula (3A).

[NOx]+2[O₂]═[CO]  (3A)

In this instance, as shown in FIG. 10, [NOx] is approximately from 1/30to 1/50 of [CO] in concentration. Thus, in FIG. 7, NOx concentrationcharacteristics with respect to the concentration of oxygen are omitted,and [NOx] of formula (3A) can be negligible. Where the concentration ofoxygen on the primary side is X1 on the fifth line L, the concentrationof carbon monoxide on the primary side Y1 will be Y1=2X1+[NOx]. Inaddition, since confirmation has been made for the predeterminedreference concentration ratio K0, which decreases the concentration ofnitrogen oxides and that of carbon monoxide on the secondary side of thecatalyst 4 to substantially zero in a range of the concentration ratio Kexceeding 1.0 up to 2.0, the fifth line L is not limited to the line Lshown in the drawing but may include any line satisfying formula (2).

Then, a predetermined reference concentration ratio K0 of oxygen,nitrogen oxides, and carbon monoxide at a point at which a sixth line Mindicating the primary characteristic curve of the burner 1 and thewater tube group 2 intersects with the fifth line L is the specificreference concentration ratio K0X. Where the concentration ratio K onthe primary side is given as the specific reference concentration ratioK0X, the catalyst 4 has such characteristics that the concentration ofnitrogen oxides and that of carbon monoxide on the secondary side of thecatalyst 4 are decreased to substantially zero. The adjustment to thereference concentration ratio K0X corresponds to Adjustment 0 of thepresent invention.

Then, the catalyst 4 has such characteristics that when theconcentration of oxygen on the primary side is made higher than thereference oxygen concentration SK corresponding to the specificreference concentration ratio K0X, oxygen is detected on the secondaryside of the catalyst 4 in a concentration depending on a differencebetween the concentration of oxygen on the primary side and thereference oxygen concentration, the concentration of carbon monoxide onthe secondary side of the catalyst 4 is decreased to substantially zero,and the concentration of nitrogen oxides on the secondary side of thecatalyst 4 is decreased to a greater extent than the concentration ofnitrogen oxides on the primary side by reduction reaction. A regioncharacterized in that oxygen is detected on the secondary side of thecatalyst 4 and the concentration of nitrogen oxides is decreased to agreater extent than the concentration of nitrogen oxides on the primaryside is referred to as a secondary NOx leakage region R1. In thesecondary NOx leakage region R1, an air ratio of the burner 1 is inexcess of 1.0.

The catalyst 4 also has such characteristics that when the concentrationof oxygen on the primary side is lower than the reference oxygenconcentration SK, carbon monoxide is detected on the secondary side ofthe catalyst 4 in a concentration depending on a difference between theconcentration of oxygen on the primary side and the reference oxygenconcentration SK, and the concentration of nitrogen oxides on thesecondary side of the catalyst 4 is decreased to substantially zero in apredetermined range. A region characterized in that carbon monoxide isdetected on the secondary side of the catalyst 4 and the concentrationof nitrogen oxides is decreased to substantially zero is referred to asa secondary CO leakage region R2. The secondary CO leakage region R2 isa region, which realizes the Adjustment 1 of the present invention, andan air ratio of the burner 1 is less than 1.0. The air ratio of theburner 1 is set in a range free of hydrocarbons but containing oxygen onthe primary side of the catalyst 4, where it is set to less than 1.0. Aregion, which combines the secondary NOx leakage region R1 with thesecondary CO leakage region R2, is referred to as a NOx/CO decreasingregion R3.

The above-mentioned characteristics of the catalyst 4 shown in FIG. 7are in agreement with the characteristics of air ratio-NOx/CO shown inFIG. 4. As apparent from FIG. 7, when the concentration of oxygen and/orthat of the carbon monoxide on the secondary side of the catalyst 4are/is detected and the air-ratio adjusting device 28 is controlled insuch a manner that the concentration of oxygen and/or that of carbonmonoxide are/is decreased to zero, the concentration ratio K on theprimary side of the catalyst 4 is controlled to the specific referenceconcentration ratio K0X, and the concentration of nitrogen oxides andthat of carbon monoxide on the secondary side of the catalyst 4 can bedecreased to substantially zero. Thus, the first control condition issatisfied, by which the second control condition is also to besatisfied.

Failure to satisfy the first control condition would result in thegeneration of unburned combustibles such as hydrocarbons. In this case,energy loss would be caused, and the catalyst 4 would be unable toattain an effective decrease in NOx.

The second control condition is necessary in decreasing theconcentration of emitted nitrogen oxides to substantially zero. It hasbeen found by experiments and theoretical consideration that in order todecrease the concentration of nitrogen oxides and that of carbonmonoxide on the secondary side of the catalyst 4 to substantially zero,a concentration ratio K, which gives ([NOx]+2[O₂])/[CO] may beapproximately 1.0 by referring to the first reaction and the secondreaction. It has been, however, confirmed that the concentration ofemitted nitrogen oxides can be decreased to substantially zero even atthe concentration ratio K of 1 or higher, that is, from 1.0 to 2.0.

Used as the sensor 7 is a zirconia type air-fuel ratio sensor which hasa resolution of emitted oxygen concentration of 50 ppm and which isexcellent in responsiveness, that is, having a response time of 2 sec orless. As shown in FIG. 8, output characteristics of the sensor 7 arethose in which an output E is given as an output related to theconcentration of oxygen on the positive side and as an output related tothe concentration of carbon monoxide or others on the negative side. Inother words, an air ratio m is calculated by referring to theconcentration of oxygen (oxygen excess region) and the concentration ofcarbon monoxide (fuel excess region) or the like to be determined, thusobtaining an output of electric current or voltage corresponding to theair ratio m. In FIG. 8, Q1 indicates an oxygen concentration detectingzone, and Q2 indicates a carbon monoxide concentration detecting zone.

Then, the air ratio control program gives control on the basis ofsignals output by the sensor 7 in such a manner that an air ratio m ofthe burner will be the reference set air ratio m0. Specifically, theprogram is constituted as follows. That is, as shown in FIG. 9, theprogram includes such control procedures that a first control zone C1 atwhich a feeding velocity V of the motor 34 (driving amount per unittime) is changed depending on a difference between an output value Efrom the sensor 7 and a set value corresponding to the set air ratio m0,and second control zones C2A and C2B at which the feeding velocity V isdivided into a first set value V1 and a second set value V2 outside thefirst control zone C1 are provided to control a driving amount of themotor 34. In FIG. 9, P1 indicates a damper opened region, and P2indicates a damper closed region.

The first control zone C1 is set by the concentration of oxygen N1 (forexample, 100 ppm) and the concentration of carbon monoxide or others N2(for example, 50 ppm), and controlled so that an air ratio will be a setair value m0, which is substantially 1, (corresponding to the referenceoxygen concentration SK).

A feeding velocity V in the first control zone C1 can be calculated bythe following formula (4). The feeding velocity V is a driving amountper unit time. A rotating angle in Step 1 of the motor 34 of Embodiment1 is 0.075 degrees, which corresponds to change in approximately 30 ppmin terms of O₂.

V=K×ΔX  (4)

where K represents a gain, and ΔX represents a difference between theoutput value of the sensor 7 and the set value.

Next, an explanation will be given for motions of the thus constitutedsteam boiler. First, combustible air (ambient air) supplied from theblower 26 is premixed with fuel gas supplied from the fuel gas supplytube 24 inside the air supply duct 27. The thus premixed air is ejectedfrom the burner 1 to the first gas duct 15 inside the storage waterheater body 3. The premixed air is ignited by ignition device (notillustrated) to burn. This burning is conducted at a low air ratio closeto 1.0.

The gas generated in accordance with this burning is in contact with anupstream water tube group 2 and cooled. Thereafter, it is treatedendothermically through heat exchange with a downstream water tube group2 to yield gas at approximately 100° C. to 350° C. The gas free ofhydrocarbons but containing oxygen, nitrogen oxides, and carbon monoxideis treated by the catalyst 4 and emitted as exhaust gas into theatmosphere from the second gas duct 17, after the concentrations ofnitrogen oxides and that of carbon monoxide are decreased tosubstantially zero.

Next, an explanation will be made for an air ratio control by theair-ratio adjusting device 28. The boiler used in the present embodimentis operated by switching high combustion to low combustion. Therefore,the damper 29 is positioned by selecting a high combustible airflowposition or a low combustible airflow position.

The damper 29 is adjusted for position by the damper position adjustingdevice 30 on the basis of instructions from the controller 8. In otherwords, the controller 8 inputs a signal for selecting the highcombustion or the low combustion and an output value corresponding to anair ratio detected by the sensor 7 to output a signal for driving themotor 34, thereby adjusting an aperture position of the damper 29. Anaperture position set for the damper 29, which is used as a set valuecorresponding to each reference set air ratio m0 on high combustion orlow combustion, is stored at the controller 8 as an initial value foreach pulse number from an original point.

First, an explanation will be given for control on high combustion. Thecontroller 8 determines whether the present aperture position of thedamper 29 is on the opening side (the side to be controlled in a closingdirection) with respect to the set aperture position or on the closingside (the side to be controlled in an opening direction) and alsocalculates the driving pulse number of the motor 34. It also determineswhether the output value belongs to the first control zone C1 or thesecond control zones C2A and C2B in FIG. 9.

Where the output value belongs to the second control zone C2A, the motor34 is driven at the first set feeding velocity V2 and also at acalculated driving pulse to close the damper 29 at a high velocity.Where it belongs to the second control zone C2B, the motor 34 is drivenat the second set feeding velocity V1 and also at a calculated drivingpulse to open the damper 29 at a high velocity. Therefore, where theoutput value is relatively distant from a set value corresponding to thereference set air ratio m0, an output value corresponding to a detectedair ratio is controlled at a high velocity so as to come closer to a setvalue corresponding to the reference set air ratio m0, thus making itpossible to give air ratio control excellent in responsiveness.

Further, where the output value belongs to the first control zone C1, afeeding velocity of the motor 34 is calculated based on formula (4)after determination of a rotational direction, and the motor 34 isdriven based on the thus calculated feeding velocity and the calculateddriving pulse. The control at the first control zone C1 is made at ahigher feeding velocity as the output value is further distant from aset value corresponding to the reference set air ratio m0. Owing to theabove-mentioned control, it is possible to smoothly bring the valuecloser to a set value corresponding to a target reference set air ratiom0. Further, a stepping motor capable of securing the control of arotational position is used and a feeding velocity is controlled so asto slow down as an output value corresponding to the detected air ratiocomes closer to a set value corresponding to the set air ratio m0, thusmaking it possible to suppress overshooting and hunting of the air ratioin the vicinity of a set value corresponding to the reference set airratio m0.

The air ratio is controlled as described above, by which an air ratio ofthe burner 1 will be a low air ratio close to 1.0 and the concentrationratio of gas on the primary side of the catalyst 4 is controlled so asto change to a less extent, thus stably satisfying formula (2). As aresult, the concentration of nitrogen oxides on the secondary side ofthe catalyst 4 can be decreased to substantially zero and that of carbonmonoxide can also be decreased to substantially zero. Where a set airratio m0 is made less than 1.0, the concentration of nitrogen oxides onthe secondary side can be decreased to substantially zero and that ofcarbon monoxide can be also decreased to a value equal to or lower thana predetermined value in a range of practical values.

EXPERIMENT 1

An explanation will be given for the result of an experiment conductedunder the following conditions, that is, a storage water heater body 3having a capacity of evaporation per unit time of 800 kg (storage waterheater body with the production type of SQ-800 manufactured by theapplicant) was assembled into a premixed burner 1 to conduct combustionat 45.2 m³N/h, and a catalyst with a volume of 10 L and an innerdiameter of 360 mm was prepared in which Pt was held therein as acatalyst activating substance at 2.0 g/L. Where the reference set airratio m0 was given as 1, the concentration of carbon monoxide, that ofnitrogen oxides, and that of oxygen on the primary side of the catalyst4 (before passage through the catalyst 4) were adjusted to 2,295 ppm, 94ppm, and 1,655 ppm, respectively, in terms of an average value for 10minutes, and those on the secondary side of the catalyst 4 (afterpassage through the catalyst 4) were adjusted to less than 13 ppm, 0.3ppm, and 100 ppm, respectively, in terms of an average value for 10minutes. In this instance, the concentration of oxygen on the secondaryside of the catalyst 4, 100 ppm, was a detection limit of oxygenconcentration. Further, temperatures of gas before and after thecatalyst 4 were 302° C. and 327° C., respectively. In the presentExperiment 1 as well as the following Experiments 2 and 3, the catalyst4 was placed slightly upstream from the feed-water preheater 20, andmeasurement instruments were placed before and after the catalyst 4. Therespective concentrations and temperatures of gas after passage throughthe catalyst 4 were measured by using an instrument (PG-250)manufactured by Horiba Ltd., and the respective concentrations beforepassage through the catalyst 4 were measured by using an instrument(COPA-2000) manufactured by Horiba Ltd. As a matter of course, hardlyany change may be found in the measurement concentration value where thecatalyst 4 is arranged in the position shown in FIG. 1.

EXPERIMENT 2

FIG. 10 shows values at each concentration ratio K at the concentrationof carbon monoxide, that of nitrogen oxides, and that of oxygen obtainedin a case where the same burner 1 and the storage water heater body 3 asthose of Experiment 1 were used to conduct combustion at the same rateas that of Experiment 1, and a catalyst was used with a volume of 10 Land an inner diameter of 360 mm was prepared in which Pd was heldtherein as a catalyst activating substance at 2.0 g/L. In this instance,the concentration of oxygen after passage through the catalyst wasmeasured by the same oxygen concentration sensor as that used inExperiment 1 and indicated as 100 ppm, even when the concentration wasactually 100 ppm or less. Temperatures of gas before and after thecatalyst 4 were in the ranges of approximately 323° C. to 325° C. andapproximately 344° C. to 346° C., respectively.

According to Embodiment 1, damper position adjusting device (air-ratioadjusting device) 30 for adjusting the ratio of combustible air to fuelis used to control the air ratio to 1.0, thus making it possible toadjust the concentration ratio of oxygen, nitrogen oxides, and carbonmonoxide on the primary side of the catalyst 4 to the specific referenceconcentration ratio K0X (Adjustment 0) and also decrease theconcentration of emitted NOx and that of emitted CO to substantiallyzero. Therefore, as compared with technologies for decreasing NOx byaddition of water/steam and those for decreasing NOx by use of adenitration agent, the present invention is able to decrease NOx and COin a simple constitution in which air-ratio adjusting device and acatalyst are used.

Further, since the air ratio is set to substantially 1.0, anenergy-saving operation can be performed. Incidentally, an ordinaryboiler operated at oxygen concentration of 4% (air ratio ofapproximately 1.235) is compared with that operated at an oxygenconcentration of 0% (air ratio of approximately 1.0) to find that theboiler efficiency is increased approximately by 1 to 2%. Nowadays, whenmeasures are required for combating global warming, an increase inboiler efficiency can make a great contribution to industries.

Further, the sensor 7 is installed on the secondary side of the catalyst4 to control an air ratio, thus making it possible to obtain a stablecontrol, as compared with a case where the sensor is installed on theprimary side of the catalyst 4 to control the air ratio. The air ratiois also controlled at a resolution of oxygen concentration of 100 ppm orlower, thus making it possible to obtain air ratio control responsivelyand stably in a region great in the amount of CO and high in the COincreasing rate in characteristics of air ratio-CO.

Further, according to Embodiment 1, since the concentration of emittednitrogen oxides can be set to be substantially zero by the catalyst 4,the corrosion of the feed-water preheater 20 by the nitrogen oxides canbe suppressed. Then, sulfur oxides are hardly contained in generated gasby using the burner 1 as a premixed gas burner, the feed-water preheater20 is corroded less by sulfuric acid, and in addition, the corrosion bynitric acid is suppressed by the catalyst 4. Consequently, a boiler inwhich the feed-water preheater 20 is corroded less by exhaust gas can beprovided. Further, in the catalyst 4, about 2,000 ppm or more of carbonmonoxide performs oxidation and reduction functions, so the heatgeneration amount thereof is larger compared with that in the PatentDocument 2 (about 400 ppm of carbon monoxide is oxidized). According tothe present embodiment, a great amount of heat can be recoveredeffectively by the feed-water preheater 20.

EMBODIMENT 21

Another Embodiment 2 of the present invention will be explained byreferring to FIG. 11 and FIG. 12. In Embodiment 2, a sensor 7 fordetecting the concentration of oxygen is installed not on the secondaryside of the catalyst 4 but on the primary side. The sensor 7 is usedexclusively as a sensor for detecting the concentration of oxygen. Then,FIG. 12 shows control characteristics of the motor 34 on the basis ofthe sensor 7. Hereinafter, an explanation will be made only for partsdifferent from those of Embodiment 1, with an explanation omitted forcommon parts.

In Embodiment 2, an air ratio is controlled indirectly by detecting theconcentration of oxygen on the primary side of the catalyst 4 by usingthe sensor 7 in such a manner that a reference set air ratio m0 is setto 1.0 (the concentration of oxygen on the secondary side of thecatalyst 4 is decreased to zero). It is now known on the basis ofvarious experiment results that where the concentration of oxygen O₂ onthe primary side of the catalyst 4 is controlled to a value of0%<O₂≦1.00%, formula (2) is satisfied and the concentration of oxygen onthe secondary side of the catalyst 4 is decreased to substantially zero.In other words, it is known that the air ratio can be set tosubstantially 1.

As shown in FIG. 12, the air ratio control program of Embodiment 2includes control procedures in which a first control zone C1 forchanging based on a value E detected by the sensor 7 (oxygenconcentration signal) a feeding velocity V of the motor 34 (drivingamount per unit time) depending on a difference between the thusdetected value and the set oxygen concentration value and second controlzones C2A and C2B for dividing the feeding velocity V into a first setvalue and a second set value, respectively, outside the first controlzone C1 are provided to control a driving amount of the motor 34.

A range in which the first control zone is set will be controlled so asto fall within a range set by oxygen concentration N1 and oxygenconcentration N2. A feeding velocity V at the first control zone will becalculated by referring to formula (4) in the same manner as inEmbodiment 1.

EMBODIMENT 3

In Embodiment 3, by referring to FIG. 13, the air ratio controller 28includes a blower motor 52 for driving the blower 26 and an inverter 53for controlling a rotating speed of the motor 52. In Embodiment 4, airratio control and concentration ratio constant control are obtained notby using the damper 29 but by using the inverter 53. The control of theblower motor 52 by the controller 8 can be constituted so as to preventthe overshooting and hunting given in FIG. 9 covering Embodiment 1. Thedamper 29 controls air flow on high combustion and on low combustion bylowering the aperture on ignition and increasing the aperture duringstable combustion after ignition. This air flow control can be obtainedby using the inverter 53. The present invention shall not be limitedthereto but may be constituted so that the air flow control on ignitionor the like is obtained either by the damper 29 or the inverter 53. InEmbodiment 3, other constitutions are similar to those of Embodiment 1,an explanation of which will be omitted here.

EMBODIMENT 4

Next, Embodiment 4 including an abnormality time control program forpreventing the emission of a great amount of carbon monoxide at anabnormal time of the catalyst 4 in the air ratio control program inExample 1 will be described with reference to FIGS. 14 and 15.Hereinafter, the configuration different from that in Example 1 will bemainly described.

Referring to FIG. 14, a second sensor 54 for detecting the abnormalityof the catalyst 4 is provided on a secondary side of the catalyst 4. Thesecond sensor 54 detects the concentration of carbon monoxide. Further,a display unit 55 is provided as notification device so as to beadjacent to the controller 8. Further, the abnormality time controlprogram includes a program for controlling the air ratio to be a secondset air ratio λ2 (for example, 1.25) so that the concentration of carbonmonoxide on the primary side of the catalyst 4 becomes a second setvalue equal to or lower than the emission reference value, in order toset the concentration of carbon monoxide on the secondary side of thecatalyst 4 to be the emission reference value (300 ppm) or less even inthe case where the catalyst 4 does not function normally.

The abnormality time control program is configured as follows. When thedetected value of the second sensor 54 exceeds a set value (for example,300 ppm) that is an abnormality determination reference value, thecontrol device first notifies abnormality by the notification unit 55,and shifts the combustion at the first set air ratio λ1 to thecombustion at the second set air ratio λ2.

The operation of Embodiment 4 will be described with reference to FIG.15. In a processing step S1 (hereinafter, a processing step SN is merelyreferred to as SN), the burner 1 effects combustion at the first set airratio λ1. In this state, in S2, it is determined if the detected valueof the second sensor 54 exceeds the set value. When NO is determined inS2, the process is returned to S1.

When the detected value exceeds a set value as a result of the decreasein the performance of the catalyst 4, YES is determined, and the processis shifted to S3. The controller 8 sends a signal to the display unit 55as the notification device, and notifies abnormality. Then, the processis shifted to S4, and the combustion at the second set air ratio λ2 isstarted from the combustion at the first set air ratio λ1. Owing to thecombustion at the second set air ratio λ2, the concentration of carbonmonoxide contained in gas on the primary side of the catalyst 4 becomesthe emission reference or less (for example, 100 to 50 ppm).Consequently, even if the catalyst 4 does not function at all, theconcentration of emitted carbon monoxide can be set to be an emissionreference value or less.

The administrator or maintenance member of the boiler of Embodiment 4recognizes that the catalyst 4 is abnormal based on the notification bythe display unit 55, and performs exchange and maintenance, therebynormalizing the function of the catalyst. After that, the combustion atthe first set air ratio λ1 can be started.

EMBODIMENT 5

Embodiment 5 of the present invention will be described based on FIG.16. In Embodiment 5, the second sensor 54 is used as a sensor fordetecting the abnormality of the first sensor 7. The second sensor 54detects the same air ratio as that of the first sensor 7.

The abnormality time control program of Embodiment 5 is executed in thecontrol procedure shown in FIG. 16. More specifically, in S11, the airratio control by the first sensor 7 is performed as described above. InS12, the abnormality determination of the first sensor 7 is performed.This determination includes the following processing. First, thedetected value of the second sensor 54 is compared with the detectedvalue of the first sensor. Then, when the second sensor 54 has a normalvalue, i.e., the second sensor 54 detects a set air ratio, whereas thefirst sensor 7 has an abnormal value, i.e., the first sensor 7 does notdetect the set air ratio, the set air ratio is changed. Then, when thefirst sensor 7 does not detect the changed air ratio, and the secondsensor 54 detects the changed air ratio, it is determined that the firstsensor 7 is abnormal.

When the abnormality is determined in S12, the air ratio control by thesecond sensor 54 is conducted in S13. Thus, the air ratio is controlledby the second sensor 54 when the first sensor 7 is abnormal. Therefore,carbon monoxide which exceeds the processing ability of the catalyst 4can be prevented from being generated owing to the abnormality of thefirst sensor 7, and as a result, the emission of a great amount ofcarbon monoxides is prevented.

The present invention shall not be limited to Embodiments 1 to 5, whichhave been explained. Since the characteristics of air ratio-NOx/CO shownin FIG. 4, for example, are different in curve and concentration value,depending on a structure of the burner 1 or the storage water heaterbody 3 used in the combustion apparatus, different characteristics maybe used. Further, in Embodiments 1 and 2, a set air ratio is 1.0 ormore. The air ratio may be a value lower than 1.0 as long as combustioncharacteristics are not affected or no hydrocarbons are contained.

Further, in Embodiment 2, an O₂ concentration sensor is used as thesensor 7 but a CO concentration sensor may be used. The damper positionadjusting device 30 can be modified in structure in various ways. Themotor 34 can also include a gear motor (not illustrated) other than astepping motor. Still further, the damper position adjusting device 30is controlled by using the single controller (controller for boiler) 8.In addition to the controller 8, another controller (not illustrated)for the damper position adjusting device 30 may be installed andconnected to the sensor 7 and the controller 8, thereby controlling anair ratio.

1. A boiler, comprising: a premixed burner for burning ahydrocarbon-containing fuel to generate a gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from the gas generated by the premixed burner;an oxidation catalyst for oxidizing carbon monoxide contained in the gasafter the passage through the water tube group by oxygen and reducingthe nitrogen oxides contained in the gas by carbon monoxide; and anair-ratio adjusting device for adjusting an air ratio of the premixedburner, wherein the premixed burner and the water tube group havecharacteristics in which a concentration ratio of oxygen, nitrogenoxides, and carbon monoxide in gas on a primary side of the oxidationcatalyst becomes a predetermined concentration ratio, assuming that theair ratio is a set air ratio, the oxidation catalyst has characteristicsin which a concentration of nitrogen oxides on a secondary side of theoxidation catalyst is decreased to substantially zero and aconcentration of carbon monoxide on the secondary side of the oxidationcatalyst is decreased to substantially zero or a predetermined value orless, assuming that the concentration ratio is the predeterminedconcentration ratio, and the predetermined concentration ratio is keptto be constant by being controlled to the set air ratio by the air-ratioadjusting device.
 2. A boiler, comprising: a premixed burner for burninga hydrocarbon-containing fuel to generate a gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from the gas generated by the premixed burner;an oxidation catalyst for oxidizing carbon monoxide contained in the gasafter the passage through the water tube group by oxygen and reducingthe nitrogen oxides contained in the gas by carbon monoxide; and anair-ratio adjusting device for adjusting an air ratio of the premixedburner, wherein the premixed burner and the water tube group havecharacteristics in which a concentration ratio K of oxygen, nitrogenoxides, and carbon monoxide in gas on a primary side of the oxidationcatalyst becomes a predetermined reference concentration ratio K0,assuming that the air ratio is a reference set air ratio, the oxidationcatalyst has characteristics in which a concentration of nitrogen oxidesand a concentration of carbon monoxide on a secondary side of theoxidation catalyst are decreased to substantially zero, assuming thatthe concentration ratio K is the predetermined reference concentrationratio K0, and the predetermined concentration ratio is kept to beconstant by being controlled to the reference set air ratio by theair-ratio adjusting device.
 3. A boiler, comprising: a premixed burnerfor burning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; and an air-ratio adjusting device for adjusting an air ratioof the premixed burner, wherein the premixed burner and the water tubegroup have characteristics in which a concentration ratio K of oxygen,nitrogen oxides, and carbon monoxide in gas on a primary side of theoxidation catalyst becomes a first predetermined concentration ratio K1,assuming that the air ratio is a first set air ratio, the oxidationcatalyst has characteristics in which a concentration of nitrogen oxideson a secondary side of the oxidation catalyst is decreased tosubstantially zero and a concentration of carbon monoxide on thesecondary side of the oxidation catalyst is decreased to a predeterminedvalue or less, assuming that the concentration ratio K is the firstpredetermined concentration ratio K1, and the predeterminedconcentration ratio is kept to be constant by being controlled to thefirst set air ratio by the air-ratio adjusting device.
 4. The boileraccording to claim 2 or 3, wherein: a formula for determining thepredetermined reference concentration ratio K0 is given as the followingformula (1); the predetermined reference concentration ratio K0satisfies the following formula (2); and the first predeterminedconcentration ratio K1 is made smaller than the reference concentrationratio K0:([NOx]+2[O₂])/[CO]=K  (1)1.0≦K=K0≦2.0  (2) where [CO], [NOx], and [O₂] represent concentrationsof carbon monoxide, nitrogen oxides, and oxygen, respectively, andsatisfying a condition of [O₂]>0.
 5. (canceled)
 6. A boiler, comprising:a premixed burner for burning a hydrocarbon-containing fuel to generatea gas free of hydrocarbon but containing oxygen, nitrogen oxides, andcarbon monoxide; a water tube group for absorbing heat from the gasgenerated by the premixed burner; an oxidation catalyst for oxidizingcarbon monoxide contained in the gas after the passage through the watertube group by oxygen and reducing the nitrogen oxides contained in thegas by carbon monoxide; a sensor for detecting an air ratio of thepremixed burner; and an air-ratio adjusting device for controlling thepremixed burner to a set air ratio based on a detected signal of thesensor, wherein the premixed burner and the water tube group havecharacteristics of air ratio-NOx/CO in which, when the air ratio isadjusted to the set air ratio by the air-ratio adjusting device, aconcentration of nitrogen oxides and a concentration of oxygen on asecondary side of the oxidation catalyst is decreased to substantiallyzero, and a concentration of carbon monoxide on the secondary side ofthe oxidation catalyst is decreased to substantially zero or apredetermined value or less.
 7. A boiler, comprising: a premixed burnerfor burning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst for oxidizing carbon monoxidecontained in the gas after the passage through the water tube group byoxygen and reducing the nitrogen oxides contained in the gas by carbonmonoxide; a sensor for detecting an air ratio of the premixed burner;and an air-ratio adjusting device for controlling the premixed burner toa set air ratio based on a detected signal of the sensor, wherein thepremixed burner and the water tube group are configured so that, whenthe air-ratio adjusting device is used to adjust the air ratio to theset air ratio, the concentration of carbon monoxide on a primary side ofthe oxidation catalyst is substantially equal to or greater than a valueobtained by adding a concentration of carbon monoxide decreased insidethe oxidation catalyst due to the oxidation to a concentration of carbonmonoxide decreased inside the catalyst due to the reduction.
 8. Aboiler, comprising: a premixed burner for burning ahydrocarbon-containing fuel to generate a gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from the gas generated by the premixed burner;an oxidation catalyst for oxidizing carbon monoxide contained in the gasafter the passage through the water tube group by oxygen and reducingthe nitrogen oxides contained in the gas by carbon monoxide; a sensorfor detecting an air ratio of the premixed burner; and an air-ratioadjusting device for controlling the premixed burner to a set air ratiobased on a detected signal of the sensor, wherein the premixed burnerand the water tube group are configured so that, when the air-ratioadjusting device is used to adjust the air ratio to the set air ratio, aconcentration ratio of the gas before flowing into the oxidationcatalyst satisfies the following formula (3):([NOx]+2[O₂])/[CO]≦2.0  (3) where [CO], [NOx], and [O₂] representconcentrations of carbon monoxide, nitrogen oxides, and oxygen,respectively, and satisfying a condition of [O₂]>0.
 9. (canceled)
 10. Aboiler, comprising: a premixed burner for burning ahydrocarbon-containing fuel to generate a gas free of hydrocarbon butcontaining oxygen, nitrogen oxides, and carbon monoxide; a water tubegroup for absorbing heat from the gas generated by the premixed burner;an oxidation catalyst for oxidizing carbon monoxide contained in the gasafter the passage through the water tube group by oxygen and reducingthe nitrogen oxides contained in the gas by carbon monoxide; and anair-ratio adjusting device for adjusting an air ratio of the premixedburner, wherein the premixed burner and the water tube group havecharacteristics of air ratio-NOx/CO on a primary side of the oxidationcatalyst regarding the gas containing oxygen, nitrogen oxides, andcarbon monoxide on the primary side of the oxidation catalyst obtainedby adjusting in a vicinity of an air ratio of 1.0 by the air-ratioadjusting device, the oxidation catalyst has characteristics of airratio-NOx/CO on a secondary side of the oxidation catalyst obtained bybringing a gas having the characteristics of air ratio-NOx/CO on theprimary side into contact with the oxidation catalyst, and the air-ratioadjusting device controls an air ratio of the premixed burner at a setair ratio at which a concentration of nitrogen oxides on the secondaryside of the oxidation catalyst is decreased to substantially zero in aNOx/CO decreasing region of the characteristics of air ratio-NOx/CO onthe secondary side.
 11. A boiler, comprising: a premixed burner forburning a hydrocarbon-containing fuel to generate a gas free ofhydrocarbon but containing oxygen, nitrogen oxides, and carbon monoxide;a water tube group for absorbing heat from the gas generated by thepremixed burner; an oxidation catalyst that is brought into contact withthe gas containing oxygen, nitrogen oxides, and carbon monoxide afterthe passage through the water tube group; and an air-ratio adjustingdevice for adjusting a ratio between a combustible air amount of thepremixed burner and a fuel amount, wherein the oxidation catalyst hascharacteristics of decreasing a concentration of carbon monoxide by:when a concentration ratio of oxygen, nitrogen oxides, and carbonmonoxide in a gas on a primary side of the oxidation catalyst at which aconcentration of nitrogen oxides and a concentration of carbon monoxideon a secondary side of the oxidation catalyst are decreased tosubstantially zero is a reference concentration ratio, and theconcentration ratio is the reference concentration ratio, concentrationsof oxygen, nitrogen oxides, and carbon monoxide on the secondary side ofthe oxidation catalyst being decreased to substantially zero; and when aconcentration of oxygen on the primary side is lower than aconcentration of the reference oxygen, concentrations of nitrogen oxidesand oxygen on the secondary side of the oxidation catalyst beingdecreased to substantially zero, and the air-ratio adjusting deviceadjusts a concentration of oxygen on the primary side of the oxidationcatalyst with respect to the concentration of the reference oxygen byadjusting the air ratio based on concentrations of oxygen and/or carbonmonoxide on the secondary side of the oxidation catalyst, and decreasesa concentration of nitrogen oxides on the secondary side of theoxidation catalyst and the concentration of carbon monoxide on thesecondary side of the oxidation catalyst to substantially zero, therebydecreasing the concentration of carbon monoxide or decreasing theconcentration of carbon monoxide to substantially zero.
 12. The boileraccording to any one of claims 1 to 3, 6 to 8, 10, and 11, comprising anitrogen oxide generation suppressing device for suppressing atemperature of the gas to suppress a concentration of nitrogen oxides toa predetermined value or less.
 13. (canceled)
 14. (canceled)